Channel state information for reference signals in a wireless communication system

ABSTRACT

According to some embodiments, a method is performed by a user equipment for reporting channel state information. The user equipment is configured with two or more channel state information reference signal, CSI-RS, resources in a CSI-RS resource set. The method comprises: selecting at least two CSI-RS resources from the CSI-RS resource set, wherein each of the at least two selected CSI-RS resources are associated to a set of spatially multiplexed layers, wherein different sets comprise different layers; determining a preferred precoder matrix for the selected CSI-RS resources; and transmitting a CSI report indicating the selected CSI-RS resources and the preferred precoder matrices. The method may further comprise calculating a channel estimate for the selected CSI-RS resources, and determining a channel quality indicator, CQI, corresponding to a hypothetical transmission from a plurality of effective channels where layers transmitted through the effective channels mutually interfere. The CSI report may indicate the determined CQI for the selected CSI-RS resources.

PRIORITY

This nonprovisional application is a U.S. National Stage Filing under 35U, C, § 371 of international Patent Application Serial No.PCT/EP2018/065557 filed Jun. 12, 2018 and entitled “CHANNEL STATEINFORMATION FOR REFERENCE SIGNALS IN A WIRELESS COMMUNICATION SYSTEM”which claims priority to U.S. Provisional Patent Application No.62/521,052 filed Jun. 16, 2017 both of which are hereby incorporated byreference in their entirety.

TECHNICAL FIELD

Particular embodiments are directed to channel state information in awireless communication system for reference signal resources, eachassociated with spatially multiplexed layers.

BACKGROUND

The next generation mobile wireless communication system (5G) or newradio (NR) supports a diverse set of use cases and a diverse set ofdeployment scenarios. The deployment scenarios include deployment atboth low frequencies (100s of MHz), similar to long term evolution(LTE), and very high frequencies (mm waves in the tens of GHz).

Similar to LTE, NR may use orthogonal frequency division multiplexing(OFDM) in the downlink (i.e., from a network node, gNB, eNB, or basestation, to a user equipment or UE). The uplink (i.e., from UE to gNB)may use both Discrete Fourier Transform (DFT)-spread OFDM and OFDM.

The basic NR physical resource can thus be seen as a time-frequency gridsimilar to the one in LTE as illustrated in FIG. 1, where each resourceelement corresponds to one OFDM subcarrier during one OFDM symbolinterval. Although a subcarrier spacing of Δf=15 kHz is shown in FIG. 1,NR supports different subcarrier spacing values. The supportedsubcarrier spacing values (also referred to as different numerologies)in NR are given by Δf=(15×2^(α)) kHz where α is a non-negative integer.

Resource allocation in LTE is typically described in terms of resourceblocks (RBs), where a resource block corresponds to one slot (0.5 ms) inthe time domain and twelve contiguous subcarriers in the frequencydomain. Resource blocks are numbered in the frequency domain, startingwith 0 from one end of the system bandwidth. Physical layer proceduresfor 3GPP LTE release 14 are specified in 3GPP TS 36.213V14.2.0. For NR,a resource block is also twelve subcarriers in frequency but may differin the time domain. A RB may also be referred to as physical RB (PRB).The 3GPP NR study on physical layer aspects is described in 3GPP TR38.802 v14.0.0.

In the time domain, downlink and uplink transmissions in NR may beorganized into equally-sized subframes similar to LTE as shown in FIG.2. In NR, the subframe length for a reference numerology of (15×2^(α))kHz is exactly 1/2^(α) ms.

Downlink transmissions are dynamically scheduled (i.e., in each subframethe gNB transmits downlink control information (DCI) about which UE datais to be transmitted to and about which resource blocks in the currentdownlink subframe the data is transmitted on). The control signaling istypically transmitted in the first one or two OFDM symbols in eachsubframe in NR. The control information is carried on a Physical ControlChannel (PDCCH) and data is carried on a Physical Downlink SharedChannel (PDSCH). A UE first detects and decodes PDCCH and if a PDCCH isdecoded successfully, it decodes the corresponding PDSCH based on thedecoded control information in the PDCCH.

Each UE is assigned a unique C-RNTI (Cell Radio Network TemporaryIdentifier) in the same serving cell. The CRC (cyclic redundancy check)bits of a PDCCH for a UE is scrambled by the UE's C-RNTI, so a UErecognizes its PDCCH by checking the C-RNTI used to scramble the CRC(cyclic redundancy check) bits of the PDCCH.

Uplink data transmissions are also dynamically scheduled using PDCCH.Similar to downlink, a UE first decodes uplink grants in PDCCH and thentransmits data over the Physical Uplink Shared Channel (PUSCH) based thedecoded control information in the uplink grant such as modulationorder, coding rate, uplink resource allocation, etc.

In LTE, semi-persistent scheduling (SPS) is supported in both uplink anddownlink. A sequence of periodic data transmissions is activated ordeactivated by a single PDCCH. There is no PDCCH transmitted for datatransmissions after activation. In SPS, the PDCCH's CRC is scrambled bya SPS-C-RNTI, which is configured for a UE if the UE supports SPS.

In addition to PUSCH, NR also supports Physical Uplink Control Channel(PUCCH). PUCCH carries uplink control information (UCI) such as HARQ(Hybrid Automatic Repeat Request) related Acknowledgement (ACK),Negative Acknowledgement (NACK), or Channel State Information (CSI)feedback.

NR includes codebook-based precoding. Multi-antenna techniques cansignificantly increase the data rates and reliability of a wirelesscommunication system. The performance is particularly improved if boththe transmitter and the receiver are equipped with multiple antennas,which results in a multiple-input multiple-output (MIMO) communicationchannel. Such systems and/or related techniques are commonly referred toas MIMO.

One component of NR is support for MIMO antenna deployments and MIMOrelated techniques. NR may support up to 8 or 16-layer downlink spatialmultiplexing for up to 32 or 64 antenna ports with channel dependentprecoding. The spatial multiplexing mode targets high data rates infavorable channel conditions. An illustration of the spatialmultiplexing operation is provided in FIG. 3.

As illustrated in FIG. 3, the information carrying symbol vector s ismultiplied by an N_(T)×r precoder matrix W, which serves to distributethe transmit energy in a subspace of the N_(T) (corresponding to N_(T)antenna ports) dimensional vector space. The precoder matrix istypically selected from a codebook of possible precoder matrices, and istypically indicated by means of a precoder matrix indicator (PMI), whichspecifies a unique precoder matrix in the codebook for a given number ofsymbol streams. The r symbols in s each correspond to a layer and r isreferred to as the transmission rank. Spatial multiplexing is achievedbecause multiple symbols can be transmitted simultaneously over the sametime/frequency resource element (TFRE). The number of symbols r istypically adapted to suit the current channel properties.

NR uses OFDM in the downlink (and OFDM or DFT precoded OFDM in theuplink). The received N_(R)×1 vector y_(n) for a certain TFRE onsubcarrier n (or alternatively data TFRE number n) is thus modeled byy _(n) =H _(n) Ws _(n) +e _(n)  Equation 1where e_(n) is a noise/interference vector obtained as realizations of arandom process. The precoder W can be a wideband precoder, which isconstant over frequency, or frequency selective.

The precoder matrix W is often chosen to match the characteristics ofthe N_(R)×N_(T) MIMO channel matrix H_(n), resulting in channeldependent precoding. This is also commonly referred to as closed-loopprecoding, and focuses the transmit energy into a subspace which isstrong in the sense of conveying much of the transmitted energy to theUE. In addition, the precoder matrix may also be selected toorthogonalize the channel, which means that after proper linearequalization at the UE, the inter-layer interference is reduced.

One example method for a UE to select a precoder matrix W is to selectthe W_(k) that maximizes the Frobenius norm of the hypothesizedequivalent channel:

$\begin{matrix}{\max\limits_{k}{{{\overset{\hat{}}{H}}_{n}W_{k}}}_{F}^{2}} & {{Equation}\mspace{14mu} 2}\end{matrix}$Where Ĥ_(n) is a channel estimate, possibly derived from CSI-RS asdescribed in more detail below. W_(k) is a hypothesized precoder matrixwith index k. Ĥ_(n)W_(k) is the hypothesized equivalent channel.

In closed-loop precoding for the NR downlink, the UE transmits, based onchannel measurements in the forward link (downlink), recommendations tothe gNodeB of a suitable precoder to use. The gNodeB configures the UEto provide feedback according to the UE's transmission mode, and maytransmit CSI-RS and configure the UE to use measurements of CSI-RS tofeed back recommended precoding matrices that the UE selects from acodebook.

A single precoder that is supposed to cover a large bandwidth (widebandprecoding) may be fed back. It may also be beneficial to match thefrequency variations of the channel and instead feed back afrequency-selective precoding report (e.g., several precoders, one persubband). This is an example of the more general case of channel stateinformation (CSI) feedback, which also encompasses feeding back otherinformation than recommended precoders to assist the eNodeB insubsequent transmissions to the UE. Such other information may includechannel quality indicators (CQIs) as well as transmission rank indicator(RI).

Given the CSI feedback from the UE, the gNodeB determines thetransmission parameters it wishes to use to transmit to the UE,including the precoding matrix, transmission rank, and modulation andcoding state (MCS). These transmission parameters may differ from therecommendations the UE makes. Therefore, a rank indicator and MCS may besignaled in downlink control information (DCI), and the precoding matrixcan be signaled in DCI or the gNodeB can transmit a demodulationreference signal from which the equivalent channel can be measured. Thetransmission rank, and thus the number of spatially multiplexed layers,is reflected in the number of columns of the precoder W. For efficientperformance, it is important to select a transmission rank that matchesthe channel properties.

LTE includes multiple transmission schemes, such as: (a) single antennaport scheme; (b) transmit diversity scheme; (c) large delay CDD (CyclicDelay Diversity) scheme; (d) closed-loop spatial multiplexing scheme;(e) multi-user MIMO (Multiple Input and Multiple Output) scheme; (f)dual layer scheme; and (g) up to 8 layer transmission scheme. Inaddition, LTE includes ten transmission modes (TMs) (i.e., Mode 1 toMode 10).

Each transmission mode is associated with a transmission scheme. A UE issemi-statically configured with one transmission mode. For eachtransmission mode, the CSI contents are generally different.

For example, TM3 is associated with large delay CDD scheme, generallyreferred to as open-loop transmission mode. In TM3, precoder matrixindication (PMI) is not reported in CSI and only one channel qualityindication (CQI) is reported regardless of rank 1 or rank 2.

TM4 is associated with close-loop spatial multiplexing scheme, generallyreferred to as close-loop transmission mode. CSI report includes PMI,rank indication (RI) and CQI.

TM9 is associated with the “up to 8 layer transmission scheme” and CSIreport in this TM includes RI, PMI and CQI. However, in LTE Rel-14,semi-open-loop transmission and an advanced CSI codebook were introducedto TMs 9 and 10. The CSI contents are different in each case. Forsemi-open-loop, either no PMI or partial PMI is fed back depending onthe number of antennas and codebooks used. For advanced codebook basedCSI, higher resolution CSI is fed back from the UE to the base stationand there are more CSI bits to feedback.

TM10 is also associated with the “up to 8-layer transmission scheme” butcan support CSI feedback for more than one serving transmission point orcell, so it is often referred as a CoMP (Coordinated MultipleTransmission Point) mode. In general, the CSI contents and payload sizeare different for different TMs.

Similar to LTE, NR transmits a unique reference signal from each antennaport at the gNB for downlink channel estimation at a UE. Referencesignals for downlink channel estimation are commonly referred to aschannel state information reference signals (CSI-RSs). For N antennaports, there will be N CSI-RS signals, each associated with one antennaport.

By measuring on CSI-RS, a UE can estimate the effective channel theCSI-RS is traversing including the radio propagation channel and antennagains at both the gNB and the UE. Mathematically, if a known CSI-RSsignal x_(i) (i=1,2, . . . , N_(tx)) is transmitted on the ith transmitantenna port at gNB, the received signal y_(j) (j=1,2, . . . , N_(rx))on the jth receive antenna port of a UE can be expressed asy _(j) =h _(i,j) x _(i) +n _(j)Where h_(i,j) is the effective channel between the ith transmit antennaport and the jth receive antenna port, n_(j) is the receiver noiseassociated with the jth receive antenna port, N_(tx) is the number oftransmit antenna ports at the gNB and and N_(rx) is the number ofreceive antenna ports at the UE.

A UE can estimate the N_(rx)×N_(tx) effective channel matrix H(H(i,j)=h_(i,j)) and thus the channel rank, precoding matrix, andchannel quality. This is achieved by using a predesigned codebook foreach rank, with each codeword in the codebook being a precoding matrixcandidate. A UE searches through the codebook to find a rank, a codewordassociated with the rank, and channel quality associated with the rankand precoding matrix to best match the effective channel. The rank, theprecoding matrix and the channel quality are reported in the form of arank indicator (RI), a precoding matrix indicator (PMI) and a channelquality indicator (CQI) as part of CSI feedback. This may be referred toas channel dependent precoding or closed-loop precoding. Such precodingtries to focus the transmit energy into a subspace which is strong inthe sense of conveying much of the transmitted energy to the UE.

A CSI-RS signal is transmitted on a set of time-frequency resourceelements (REs) associated with an antenna port. For channel estimationover a system bandwidth, CSI-RS is typically transmitted over the entiresystem bandwidth. The set of REs used for CSI-RS transmission isreferred to as a CSI-RS resource. From a UE point of view, an antennaport is equivalent to a CSI-RS that the UE uses to measure the channel.Up to 32 (i.e., N_(tx)=32) antenna ports are supported in NR and thus 32CSI-RS signals can be configured for a UE in a CSI-RS resource.

NR supports two types of CSI feedback for closed-loop transmission(i.e., Type I and Type II). Type I is codebook based PMI feedback withnormal resolution targeting single user MIMO (SU-MIMO) transmissions.Type II is an enhanced CSI feedback with higher resolution targetingmulti-user MIMO (MU-MIMO) transmissions.

Two different codebooks may be designed for the two feedback types. TypeII feedback may use a larger number of bits for PMI feedback than inType I.

In LTE, UEs can be configured to report CSI in periodic or aperiodicreporting modes. Periodic CSI reporting is carried on PUCCH whileaperiodic CSI is carried on PUSCH. PUCCH is transmitted in a fixed orconfigured number of PRBs and using a single spatial layer withquadrature phase-shift keying (QPSK) modulation. PUSCH resourcescarrying aperiodic CSI reporting are dynamically allocated throughuplink grants carried over PDCCH or enhanced PDCCH (EPDCCH), and canoccupy a variable number of PRBs, use modulation states such as QPSK,16QAM, and 64QAM, as well as multiple spatial layers.

In LTE, a periodic CSI report can be scheduled for the same subframes asthose containing SPS PUSCH, in which case the periodic CSI reports arepiggy backed on PUSCH. This allows periodic CSI to be transmitted usinglink adaptation, and so periodic CSI can be transmitted in a morespectrally efficient manner than on PUCCH (which always uses QPSK with afixed number of resources). However, periodic CSI reports are formedsuch that they fit in the small payload of PUCCH, and so may carry lessinformation even when they are piggy backed on PUSCH, for example, bythe use of codebook subsampling. By contrast, aperiodic CSI reporting onPUSCH uses the full resolution of the CSI feedback, and is notsubsampled. Furthermore, periodic CSI reporting in LTE requires that atleast one PUCCH resource be configured for the UE, which is a waste ofPUCCH resources which are reserved and may be unused even if theperiodic CSI is always carried on PUSCH. Therefore, while LTE cantransmit periodic CSI on PUSCH with semi-persistent resource allocation,such CSI is generally less accurate than aperiodic CSI on PUSCH

In LTE, the PDCCH uplink grant allocates a single resource for allcontent to be carried on the PUSCH, including UL-SCH, CSI (including RI,CRI, RPI, CQI, and PMI), and HARQ-ACK. Because the size of the messageis determined according to the reported RI, CRI, and/or RPI when CSI ispiggy backed on PUSCH, the eNB does not know at the time of the uplinkgrant what the size of the uplink CSI will be. The eNB must thereforeallocate extra resources to ensure that both the CSI and the othercontent will fit on the PUSCH resource. It should also be noted that CSIon PUSCH always carries complete CSI messages for each cell, CSIprocess, and/or eMIMO-Type. All configured parameters (i.e., one or moreof RI, CRI, RPI, CQI, PMI) to be reported for the cell, CSI process,and/or eMIMO-type are reported together in one transmission on PUSCH.

The UE is generally required to update each new CSI report whether it isreported periodically or aperiodically. However, if the number of CSIreports to be produced is greater than the number of CSI processes, theUE is not required to update the CSI report to limit the UE computationcomplexity. This does not, however, mean that the UE is forbidden fromupdating the report, and so whether a CSI report will be identical to aprior transmitted report in this case is not known.

In LTE, a UE can be configured with multiple CSI-RS resources fordownlink CSI acquisition purposes if Class B eMIMO-Type is used. ACSI-RS resource is defined by a certain number of CSI-RS at a certainposition in the time-frequency resource grid and can be associated witha certain quasi-colocation (QCL) assumption and relative power leveltowards another reference signal. In Class B operation, the CSI-RS ineach CSI-RS resource are typically precoded with different precodingweights so as to form different transmit beams. As part of the CSIreporting procedure, the UE may select a preferred CSI-RS resource,corresponding to a preferred transmit beam, with a CSI-RS resourceindicator (CRI). The UE then determines an appropriate PMI, RI andcorresponding CQI for the selected CSI-RS resource by performing aprecoder search. Thus, the UE first selects the best CSI-RS resource andthen applies a precoder codebook within the selected CSI-RS resource.

In NR, a UE can be configured with N≥1 CSI reporting settings, M≥1Resource settings, and 1 CSI measurement setting, where the CSImeasurement setting includes L≥1 links and the value of L may depend onthe UE capability. At least the following configuration parameters aresignaled via RRC at least for CSI acquisition.

N, M, and L are indicated either implicitly or explicitly. Each CSIreporting setting may include at least: (a) reported CSI parameter(s);(b) CSI Type (I or II) if reported; (c) codebook configuration includingcodebook subset restriction; (d) time-domain behavior; (e) frequencygranularity for CQI and PMI; and (f) measurement restrictionconfigurations.

Each Resource setting may include: (a) a configuration of S≥1 CSI-RSresource set(s); (b) a configuration of Ks≥1 CSI-RS resources for eachset s, including at least: mapping to REs, the number of ports,time-domain behavior, etc.; (c) time domain behavior, such as aperiodic,periodic or semi-persistent; and (d) RS type which encompasses at leastCSI-RS.

Each of the L links in CSI measurement setting may include: (a) CSIreporting setting indication, (b) resource setting indication; and (c)quantity to be measured (either channel or interference). One CSIreporting setting can be linked with one or multiple resource settings.Multiple CSI reporting settings can be linked.

At least the following may be dynamically selected by L1 or L2signaling, if applicable: (a) one or multiple CSI reporting settingswithin the CSI measurement setting; (b) one or multiple CSI-RS resourcesets selected from at least one resource setting; and (c) one ormultiple CSI-RS resources selected from at least one CSI-RS resourceset.

LTE control signaling can be carried in a variety of ways, includingcarrying control information on PDCCH or PUCCH, embedded in the PUSCH,in MAC control elements (MAC CEs), or in RRC signaling. Each of thesemechanisms may be customized to carry a particular kind of controlinformation.

Control information carried on PDCCH, PUCCH, or embedded in(‘piggybacked on’) PUSCH is physical layer related control information,such as downlink control information (DCI), uplink control information(UCI), as described in 3GPP TS 36.211, 36.212, and 36.213. DCI isgenerally used to instruct a UE to perform some physical layer function,providing the needed information to perform the function. UCI generallyprovides the network with needed information, such as HARQ-ACK,scheduling request (SR), channel state information (CSI), including CQI,PMI, RI, and/or CRI.

UCI and DCI can be transmitted on a subframe-by-subframe basis, andsupport rapidly varying parameters, including those that can vary with afast fading radio channel. Because UCI and DCI can be transmitted inevery subframe, UCI or DCI corresponding to a given cell tend to be onthe order of tens of bits to limit the amount of control overhead.

Control information carried in MAC CEs is carried in MAC headers on theuplink and downlink shared transport channels (UL-SCH and DL-SCH), asdescribed in 3GPP TS 36.321. Because a MAC header does not have a fixedsize, control information in MAC CEs can be sent when it is needed, anddoes not necessarily represent a fixed overhead.

Furthermore, MAC CEs can carry larger control payloads efficiently,because they are carried in UL-SCH or DL-SCH transport channels, whichbenefit from link adaptation, HARQ, and can be turbo coded. MAC CEs areused to perform repetitive tasks that use a fixed set of parameters,such as maintaining timing advance or buffer status reporting, but thesetasks generally do not require transmission of a MAC CE on asubframe-by-subframe basis. Consequently, channel state informationrelated to a fast fading radio channel, such as PMI, CQI, RI, and CRIare not carried in MAC CEs in LTE up to Rel-14.

The embodiments described herein may be used with two-dimensionalantenna arrays and some of the presented embodiments use such antennas.Such antenna arrays may be (partly) described by the number of antennacolumns corresponding to the horizontal dimension N_(h), the number ofantenna rows corresponding to the vertical dimension N, and the numberof dimensions corresponding to different polarizations N_(p). The totalnumber of antennas is thus N=N_(h)N_(v)N_(p). The concept of an antennais non-limiting in the sense that it can refer to any virtualization(e.g., linear mapping) of the physical antenna elements. For example,pairs of physical sub-elements could be fed the same signal, and thusshare the same virtualized antenna port. An example of a 4×4 array withcross-polarized antenna elements is illustrated In FIG. 4.

FIG. 4 illustrates a two-dimensional antenna array of cross-polarizedantenna elements (N_(p)=2), with N_(h)=4 horizontal antenna elements andN_(v)=4 vertical antenna elements. Precoding may be interpreted asmultiplying the signal with different beamforming weights for eachantenna prior to transmission. A typical approach is to tailor theprecoder to the antenna form factor (i.e., taking into account N_(h),N_(v) and N_(p) when designing the precoder codebook). Suchtwo-dimensional codebooks may not strictly relate vertical or horizontaldimensions to the dimensions that antenna ports are associated with.Therefore, two-dimensional codebooks can be considered to have a firstand a second number of antenna ports N₁ and N₂, wherein N₁ cancorrespond to either the horizontal or vertical dimension, and N₂corresponds to the remaining dimension. That is, if N₁=N_(h), thenN₂=N_(v), while if N₁=N_(v), then N₂=N_(h). Similarly, two-dimensionalcodebooks may not strictly relate antenna ports to polarization, and maybe designed with cophasing mechanisms used to combine two beams or twoantenna ports, as described in the following.

Some transmitters may include multi-panel antenna arrays. When buildingvery large antenna arrays, it can be challenging to fit in all thehardware components into a single antenna panel. One building practiceis to use a modular approach and construct a multi-panel antenna arrayconsisting of multiple antenna panels (as defined above). In the generalcase, the spacing between the right-most antenna element of a firstpanel and the left-most antenna element of a second panel placed to theright of the first panel can be larger than the spacing between antennaelements within a panel, corresponding to a non-uniform multi-panelarray. It is generally assumed that the tight calibration required forseamless coherent transmission between antenna elements is only donewithin each panel, and so, different panels of the multi-panel array canbe uncalibrated. There may thus exist a frequency offset, timingmisalignment, and a LO phase offset between the panels.

A multi-panel array can, for example, be parametrized in the number ofvertical panels M_(g), the number of horizontal panels N_(y) and size ofthe constituent panels M, N, P. An example of a multi-panel antennaarray is given in FIG. 5, which illustrates a size M_(g)×N_(g)=2×2multi-panel antenna array consisting of (M,N,P)=(4,4,2) panels.

A common type of precoding is to use a DFT-precoder, where the precodervector used to precode a single-layer transmission using asingle-polarized uniform linear array (ULA) with N₁ antennas is definedas

$\begin{matrix}{{w_{1D}\left( {l,N_{1},O_{1}} \right)} = {\frac{1}{\sqrt{N_{1}}}\begin{bmatrix}e^{{j2\pi} \cdot 0 \cdot \frac{l}{O_{1}N_{1}}} \\e^{{j2\pi} \cdot 1 \cdot \frac{l}{O_{1}N_{1}}} \\\vdots \\e^{{j2\pi} \cdot {({N_{1} - 1})} \cdot \frac{l}{O_{1}N_{1}}}\end{bmatrix}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$where l=0,1, . . . O₁N₁−1 is the precoder index and O₁ is an integeroversampling factor.

A precoder for a dual-polarized uniform linear array (ULA) with N₁antennas per polarization (and so 2N₁ antennas in total) can besimilarly defined as

$\begin{matrix}{{w_{{1D},{DP}}\left( {l,N_{1},O_{1}} \right)} = {\begin{bmatrix}{w_{1D}(l)} \\{e^{j\phi}{w_{1D}(l)}}\end{bmatrix} = {\begin{bmatrix}{w_{1D}(l)} & 0 \\0 & {w_{1D}(l)}\end{bmatrix}\begin{bmatrix}1 \\e^{j\phi}\end{bmatrix}}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$where e^(jϕ) is a cophasing factor between the two polarizations thatmay for instance be selected from a QPSK alphabet

$\phi \in {\left\{ {0,\frac{\pi}{2},\pi,\frac{3\pi}{2}} \right\}.}$

A corresponding precoder vector for a two-dimensional uniform planararrays (UPA) with N₁×N₂ antennas can be created by taking the Kroneckerproduct of two precoder vectors as w_(2D) (l,m)=w_(1D) (l, N₁,O₁)⊗w_(1D) (m, N₂, O₂), where O₂ is an integer oversampling factor inthe N₂ dimension. Each precoder w_(2D) (l,m) forms a DFT beam, and allthe precoders {w_(2D) (l,m), l=0, . . . , N₁O₁−1; m=0, . . . , N₂O₂−1}form a grid of DFT beams. An example is shown in FIG. 6, where (N₁,N₂)=(4,2) and (O₁, O₂)=(4,4). Herein, the terms “DFT beams” and “DFTprecoders” may be used interchangeably.

More generally, a beam with an index pair (l,m) can be identified by thedirection in which the greatest energy is transmitted when precodingweights w_(2D) (l,m) are used in the transmission. Also, a magnitudetaper can be used with DFT beams to lower the beam's sidelobes. Aone-dimensional DFT precoder along N₁ and N₂ dimensions with magnitudetapering can be expressed as

${{W_{1D}\left( {l,N_{1},O_{1},\beta} \right)} = {\frac{1}{\sqrt{N_{1}}}\begin{bmatrix}{\beta_{0}e^{{j2\pi} \cdot 0 \cdot \frac{l}{O_{1}N_{1}}}} \\{\beta_{1}e^{{j2\pi} \cdot 1 \cdot \frac{l}{O_{1}N_{1}}}} \\\vdots \\{\beta_{N_{1}­1}e^{{{j2\pi} \cdot {({N_{1}­1})}}\frac{l}{O_{1}N_{1}}}}\end{bmatrix}}},{{W_{1D}\left( {m,N_{2},O_{2},\gamma} \right)} = {\frac{1}{\sqrt{N_{2}}}\begin{bmatrix}\begin{matrix}{\gamma_{0}e^{{j2\pi} \cdot 0 \cdot \frac{m}{O_{2}N_{2}}}} \\{\gamma_{1}e^{{j2\pi} \cdot 1 \cdot \frac{m}{O_{2}N_{2}}}} \\\vdots\end{matrix} \\{\gamma_{N_{2}­1}e^{{{j2\pi} \cdot {({N_{2}­1})}}\frac{m}{O_{2}N_{2}}}}\end{bmatrix}}}$where 0<β_(i), γ_(k)≤1 (i=0,1, . . . , N₁−1; k=0,1, . . . , N₂−1) areamplitude scaling factors. β_(i)=1, γ_(k)=1 (i=0,1, . . . , N₁−1; k=0,1,. . . , N₂−1) correspond to no tapering.

DFT beams (with or without a magnitude taper) have a linear phase shiftbetween elements along each of the two dimensions. Without loss ofgenerality, the elements of w(l,m) may be ordered according tow(l,m)=w_(1D) (l, N₁, O₁, β)⊗w_(1D) (m, N₂, O₂, γ) such that adjacentelements correspond to adjacent antenna elements along dimension N₂, andelements of w(l,m) spaced N₂ apart correspond to adjacent antennaelements along dimension N₁. Then the phase shift between two elementsw_(s) ₁ (l,m) and w_(s) ₂ (l,m) of w(l,m) can be expressed as:

${w_{s_{2}}\left( {l,m} \right)} = {{w_{s_{1}}\left( {l,m} \right)} \cdot \left( \frac{\alpha_{s_{2}}}{\alpha_{s_{1}}} \right) \cdot e^{j2{\pi{({{{({k_{1} - i_{1}})}\Delta_{1}} + {{({k_{2} - i_{2}})}\Delta_{2}}})}}}}$where: (a) s₁=i₁N₂+i₂ and s₂=k₁N₂+k₂ (with 0≤i₂<N₂, 0≤i₁<N₁, 0≤k₂<N₂,and 0≤k₁<N₁) are integers identifying two entries of the beam w(l,m) sothat (i₁, i₂) indicates to a first entry of beam w(l,m) that is mappedto a first antenna element (or port) and (k₁, k₂) indicates to a secondentry of beam w(l,m) that is mapped to a second antenna element (orport); (b) α_(s) ₁ =β_(i) ₁ γ_(i) ₂ and α_(s) ₂ =β_(k) ₁ γ_(k) ₂ arereal numbers, and α_(i)≠1 (i=s₁, s₂) if magnitude tapering is used;otherwise α_(i)=1;

${(c)\mspace{14mu}\Delta_{1}} = \frac{l}{O_{1}N_{1}}$is a phase shift corresponding to a direction along an axis (e.g., thehorizontal axis or azimuth); and

$(d)\mspace{14mu}{\Delta_{2} = \frac{m}{O_{2}N_{2}}}$is a phase shift corresponding to direction along an axis (e.g., thevertical axis or elevation).

Therefore, a kth beam d(k) formed with precoder W(l_(k), m_(k)) can alsobe referred to by the corresponding precoder W(l_(k),m_(k)), i.e.d(k)=W(l_(k),m_(k)). Thus, a beam d(k) can be described as a set ofcomplex numbers, each element of the set being characterized by at leastone complex phase shift such that an element of the beam is related toany other element of the beam where d_(n)(k)=d_(i)(k)α_(i,n)e^(j2π(pΔ)^(1,k) ^(+wΔ) ^(2,k) ⁾=d_(i)(k)α_(i,n)(e^(j2π66) ^(1,k) )^(p)(e^(j2πΔ)^(2,k)) ^(q), where d_(i)(k) is the ith element of a beam d(k), α_(i,n)is a real number corresponding to the i^(th) and n^(th) elements of thebeam d(k); p and q are integers; and Δ_(1,k) and Δ_(2,k) are realnumbers corresponding to a beam with index pair (l_(k), m_(k)) thatdetermine the complex phase shifts e^(j2πΔ) ^(1,k) , and e^(j2πΔ) ^(2,k), respectively. Index pair (l_(k), m_(k)) corresponds to a direction ofarrival or departure of a plane wave when beam d(k) is used fortransmission or reception in a UPA or ULA. A beam d(k) can be identifiedwith a single index k where=l_(k)+N₁O₁m_(k), i.e, along vertical or N₂dimension first, or alternatively k=N₂O₂l_(k)+m_(k), i.e. alonghorizontal or N₁ dimension first.

Extending the precoder for a dual-polarized ULA may then be done as

$\begin{matrix}{{w_{{2D},{DP}}\left( {l,m,\phi} \right)} = {{\begin{bmatrix}1 \\e^{j\phi}\end{bmatrix} \otimes {w_{2D}\left( {l,m} \right)}} = {\begin{bmatrix}{w_{2D}\left( {l,m} \right)} \\{e^{j\phi}{w_{2D}\left( {l,m} \right)}}\end{bmatrix} = {\begin{bmatrix}{w_{2D}\left( {l,m} \right)} & 0 \\0 & {w_{2D}\left( {l,m} \right)}\end{bmatrix}\begin{bmatrix}1 \\e^{j\phi}\end{bmatrix}}}}} & {{Equation}\mspace{14mu} 5}\end{matrix}$

A precoder matrix W_(2D,DP) for multi-layer transmission may be createdby appending columns of DFT precoder vectors asW _(2D,DP) ^((R))=[w _(2D,DP)(l ₁ ,m ₁,ϕ₁)w _(2D,DP)(l ₂ ,m ₂,ϕ₂) . . .w _(2D,DP)(l _(R) ,m _(R),ϕ_(R))]where R is the number of transmission layers, i.e. the transmissionrank.

In a special case for a rank-2 DFT precoder, m₁=m₂=m and l₁=l₂=l,

$\begin{matrix}{{W_{{2D},{DP}}^{(2)}\left( {l,m,\phi_{1},\phi_{2}} \right)} = {\left\lbrack {{w_{{2D},{DP}}\left( {l,m,\phi_{1}} \right)}{w_{{2D},{DP}}\left( {l,m,\phi_{2}} \right)}} \right\rbrack = {\begin{bmatrix}{w_{2D}\left( {l,m} \right)} & 0 \\0 & {w_{2D}\left( {l,m} \right)}\end{bmatrix}\begin{bmatrix}1 & 1 \\e^{{j\phi}_{1}} & e^{{j\phi}_{2}}\end{bmatrix}}}} & {{Equation}\mspace{14mu} 6}\end{matrix}$

For each rank, all the precoder candidates form a “precoder codebook” ora “codebook”. A UE can first determine the rank of the estimateddownlink wideband channel based on CSI-RS. After the rank is identified,for each subband the UE then searches through all the precodercandidates in a codebook for the determined rank to find the bestprecoder for the subband. For example, in case of rank=1, the UE wouldsearch through W_(2D,DP) (k,l,ϕ) for all the possible (k,l,ϕ) values. Incase of rank=2, the UE would search through W_(2D,DP) ⁽²⁾(k,l,ϕ₁,ϕ₂) forall the possible (k,l,ϕ₁,ϕ₂) values.

SUMMARY

A particular problem is how to efficiently support channel stateinformation for a wide range of antenna configurations in a flexibleway, yet avoid specifying separate channel state information (CSI)feedback format for each use case related to the various antennaconfigurations. In the downlink, a user equipment (UE) may be served bya transmit/receive point (TRP) equipped with multiple antenna panels, orit may be served by multiple TRPs, each equipped with one or moreantenna panels. The panels may use different port layouts and would thusbenefit from being associated with different precoder codebooks. If asingle CSI-RS resource comprises the ports from multiple panels, avariety of different precoder codebooks corresponding to differentcombinations of panel sizes may have to be specified, which isinfeasible.

Particular embodiments obviate the problems described above. Theembodiments described herein include a user equipment (UE) configured tomeasure on multiple CSI-RS resources, where each resource corresponds toa separate transmit/receive point (TRP) or antenna panel. The UE mayselect a number of channel state information reference signal (CSI-RS)resources to participate in a non-coherent joint transmission, as wellas a preferred precoder matrix for each CSI-RS resource, on the basisthat layers corresponding to different CSI-RS resources mutuallyinterfere.

According to some embodiments, a method is performed by a user equipmentfor reporting channel state information in a wireless communicationsystem. The user equipment is configured with two or more CSI-RSresources in a CSI-RS resource set. The method comprises selecting atleast two CSI-RS resources from the CSI-RS resource set, wherein each ofthe at least two selected CSI-RS resources are associated to a set ofspatially multiplexed layers, wherein different sets comprise differentlayers, determining a preferred precoder matrix for each of the selectedCSI-RS resources, and transmitting a CSI report indicating each of theselected CSI-RS resources and the determined preferred precoder matrixfor each of the selected CSI-RS resources.

In some aspects, a first one of the selected CSI-RS resources comprisesa first set of layers and a second one of the selected CSI-RS resourcescomprises a second set of layers wherein the first set of layers and thesecond set of layers are different and the first and second set oflayers mutually interfere.

The method may further comprise calculating a channel estimate for eachof the selected CSI-RS resources and determining a channel qualityindicator (CQI) corresponding to a hypothetical transmission from aplurality of effective channels. The effective channels depend on thepreferred precoder matrices and channel estimate for each of theselected CSI-RS resources. Layers transmitted through the effectivechannels mutually interfere. The CSI report may further indicate thedetermined CQI for each of the selected CSI-RS resources.

In particular embodiments, each CSI-RS resource is associated with atleast one of a number of antenna ports (P), a multi-panel antenna arrayport layout parametrized by a number of vertical panels (M_(g)) and anumber of horizontal panels (N_(g)), and a precoder codebook. EachCSI-RS resource may be associated with a different quasi co-location(QCL) assumption.

In particular embodiments, the CSI-RS carried in each CSI-RS resource ofthe set of CSI-RS resources is transmitted from different antennasubsets. The different antenna subsets may belong to differenttransmission points or may belong to different antenna panels of thesame transmission point.

In particular embodiments, the method further comprises obtaining a CSIreport configuration comprising possible hypotheses for combinations ofone or more CSI-RS resources. Selecting the at least two CSI-RSresources comprises selecting the at least two CSI-RS resourcesaccording to a selected one of the possible hypotheses. The CSI reportindicates the selected CSI-RS resources by indicating the selectedpossible hypothesis.

In particular embodiments, transmitting the CSI report comprisestransmitting a single message to a network node. In particularembodiments, transmitting the CSI report comprises transmitting a firstmessage associated with one of the selected CSI-RS resources andtransmitting a second message associated with a second one of theselected CSI-RS resources. The first message may be transmitted to afirst transmission point and the second message may be transmitted to asecond transmission point.

In particular embodiments, the determined preferred precoder matrix forat least one selected CSI-RS comprises a first preferred precoder matrixfor a first subband. The method may further comprise determining, forthe at least one selected CSI-RS resource, a second preferred precodermatrix for a second subband. The CSI report indicates the first andsecond preferred precoder matrix for the at least one selected CSI-RSresource.

In particular embodiments, the method further comprises receiving anindication of a codeword-to-layer mapping for use when determining theCQI.

According to some embodiments, a user equipment is capable of reportingchannel state information in a wireless communication system. The userequipment is configured with two or more CSI-RS resources in a CSI-RSresource set. The user equipment comprises processing circuitry (1020)operable to select at least two CSI-RS resources from the CSI-RSresource set, determine a preferred precoder matrix for each of theselected CSI-RS resources, and transmit a CSI report indicating each ofthe selected CSI-RS resources and the determined preferred precodermatrix for each of the selected CSI-RS resources. The processingcircuitry may be further operable to calculate a channel estimate foreach of the selected CSI-RS resources and determine a CQI correspondingto a hypothetical transmission from a plurality of effective channels.The effective channels depend on the preferred precoder matrices andchannel estimate for each of the selected CSI-RS resources. Layerstransmitted through the effective channels mutually interfere. The CSIreport further indicates the determined CQI for each of the selectedCSI-RS resources.

A particular advantage is that the precoder matrix and CQI associatedwith a CSI-RS may be optimized for the particular CSI-RS. The CSI reportmay include more than one association of precoder matrix and CQI with aCSI-RS and each one may be optimized for the associated CSI-RS andassociated transmission point or antenna panel.

In particular embodiments, the processing circuitry is further operableto obtain a CSI report configuration comprising possible hypotheses forcombinations of one or more CSI-RS resources. The processing circuitryis operable to select the at least two CSI-RS resources by selecting theat least two CSI-RS resources according to a selected one of thepossible hypotheses. The CSI report indicates the selected CSI-RSresources by indicating the selected possible hypothesis.

A particular advantage is that processor complexity and signaling may bereduced. For example, the set of possible hypotheses may be less thanthe maximum available hypotheses, thus reducing the number of hypothesesavailable to the user equipment and simplifying the selection process.Also, signaling a hypothesis indicator uses fewer signaling bits thansignaling identifiers of all the resources included in the particularhypothesis.

In particular embodiments, the processing circuitry is operable totransmit the CSI report by transmitting a single message to a networknode. The processing circuitry may be operable to transmit the CSIreport by transmitting a first message associated with one of theselected CSI-RS resources and transmitting a second message associatedwith a second one of the selected CSI-RS resources. The first messagemay be transmitted to a first transmission point and the second messagemay be transmitted to a second transmission point.

In particular embodiments, the determined preferred precoder matrix forat least one selected CSI-RS comprises a first preferred precoder matrixfor a first subband. The processing circuitry is further operable todetermine, for the at least one selected CSI-RS resource, a secondpreferred precoder matrix for a second subband. The CSI report indicatesthe first and second preferred precoder matrix for the at least oneselected CSI-RS resource.

In particular embodiments, the processing circuitry is further operableto receive an indication of a codeword-to-layer mapping for use whendetermining the CQI.

According to some embodiments, a method performed by a network node of awireless communication system comprises transmitting, to a userequipment, a first CSI-RS in a first CSI-RS resource of a set of atleast two CSI-RS resources from a first antenna subset and a secondCSI-RS in a second CSI-RS resource in the set of at least two CSI-RSresources from a second antenna subset, wherein the first and secondantenna subsets comprise a first and second set of spatially multiplexedlayers, respectively. and wherein the first and second set of layers aredifferent, and receiving, from the user equipment, a CSI reportcomprising a first preferred precoder matrix associated with the firstCSI-RS resource and a second preferred precoder matrix associated withthe second CSI-RS resource. The CSI report may further comprise a CQIassociated with each of the first and second preferred precodermatrices.

In some aspects a transmission associated with a first one of theselected CSI-RS resources comprises a first set of layers and atransmission associated with a second one of the selected CSI-RSresources comprises a second set of layers wherein the first set oflayers and the second set of layers are different and the first andsecond set of layers mutually interfere.

In particular embodiments, the antenna subsets belong to differenttransmission points or belong to different antenna panels of the sametransmission point.

In particular embodiments, receiving the CSI report comprises receivinga first message that includes the first preferred precoder matrixassociated with the first CSI-RS resource and receiving a second messagethat includes the second preferred precoder matrix associated with thesecond CSI-RS resource.

In particular embodiments, the method further comprises transmitting, tothe user equipment, a CSI report configuration comprising possiblehypotheses for combinations of CSI-RS resources. The method may furthercomprise transmitting, to the user equipment, an indication on acodeword-to-layer mapping for use when determining the CQI.

According to some embodiments, a network node of a wirelesscommunication system comprises processing circuitry operable totransmit, to a user equipment, a first CSI-RS in a first CSI-RS resourceof a set of at least two CSI-RS resources from a first antenna subsetand a second CSI-RS in a second CSI-RS resource in the set of at leasttwo CSI-RS resources from a second antenna subset, wherein the first andsecond antenna subsets comprise a first and second set of spatiallymultiplexed layers, respectively. and wherein the first and second setof layers are different, and receive, from the user equipment, a CSIreport comprising at least two preferred precoder matrices eachassociated with a CSI-RS resource of the transmitted set of CSI-RSresources. The CSI report may further comprise a CQI associated witheach of the first and second preferred precoder matrices.

In particular embodiments, the antenna subsets belong to differenttransmission points or belong to different antenna panels of the sametransmission point.

In particular embodiments, the processing circuitry is operable toreceive the CSI report by receiving a first message that includes thefirst preferred precoder matrix associated with the first CSI-RSresource and receiving a second message that includes the secondpreferred precoder matrix associated with the second CSI-RS resource.

In particular embodiments, the processing circuitry is further operableto transmit, to the user equipment, a CSI report configurationcomprising possible hypotheses for combinations of CSI-RS resources. Theprocessing circuitry may be further operable to transmit, to the userequipment, an indication on a codeword-to-layer mapping for use whendetermining the CQI.

According to some embodiments, a user equipment is capable of reportingchannel state information in a wireless communication system. The userequipment is configured with two or more CSI-RS resources in a CSI-RSresource set. The user equipment comprises a determining module (1052)and a transmitting module (1054). The determining module is operable toselect at least two CSI-RS resources from the CSI-RS resource set anddetermine a preferred precoder matrix for each of the selected CSI-RSresources. The transmitting module is operable to transmit a CSI reportindicating each of the selected CSI-RS resources and the determinedpreferred precoder matrix for each of the selected CSI-RS resources.

According to some embodiments, a network node of a wirelesscommunication system comprises a transmitting module and a receivingmodule. The transmitting module is operable to transmit, to a userequipment, a first channel CSI-RS in a first CSI-RS resource of a set ofat least two CSI-RS resources from a first antenna subset and a secondCSI-RS in a second CSI-RS resource in the set of at least two CSI-RSresources from a second antenna subset. The receiving module is operableto receive, from the user equipment, a CSI report comprising a firstpreferred precoder matrix associated with the first CSI-RS resource anda second preferred precoder matrix associated with the second CSI-RSresource.

Also disclosed is a computer program product. The computer programproduct comprises instructions stored on non-transient computer-readablemedia which, when executed by a processor, perform the steps ofselecting at least two CSI-RS resources from a CSI-RS resource set,determining a preferred precoder matrix for each of the selected CSI-RSresources, and transmitting a CSI report indicating each of the selectedCSI-RS resources and the determined preferred precoder matrix for eachof the selected CSI-RS resources. The instructions may be furtheroperable to perform the steps of calculating a channel estimate for eachof the selected CSI-RS resources, and determining a CQI corresponding toa hypothetical transmission from a plurality of effective channels. Theeffective channels depend on the preferred precoder matrices and channelestimate for each of the selected CSI-RS resources. Layers transmittedthrough the effective channels mutually interfere.

Another computer program product comprises instructions stored onnon-transient computer-readable media which, when executed by aprocessor, perform the steps of transmitting, to a user equipment, afirst CSI-RS in a first CSI-RS resource of a set of at least two CSI-RSresources from a first antenna subset and a second CSI-RS in a secondCSI-RS resource in the set of at least two CSI-RS resources from asecond antenna subset, and receiving, from the user equipment, a CSIreport comprising a first preferred precoder matrix associated with thefirst CSI-RS resource and a second preferred precoder matrix associatedwith the second CSI-RS resource. The CSI report may further comprise aCQI associated with each of the first and second preferred precodermatrices.

A particular advantage of some embodiments is that a UE may jointlyselect a preferred precoder matrix and rank for each TRP/panel andcalculate an appropriate channel quality indicator (CQI) under anon-coherent joint transmission (NC-JT) assumption. By associating eachCSI-RS resource with a separate precoder codebook, particularembodiments may support a variety of different port layouts and antennadeployments.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments and their featuresand advantages, reference is now made to the following description,taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates long term evolution (LTE) physical resources;

FIG. 2 illustrates an example LTE time-domain structure with 15 kHzsubcarrier spacing;

FIG. 3 is an example transmission structure of precoded spatialmultiplexing in New Radio (NR);

FIG. 4 is an example two-dimensional array of cross-polarized antennaelements;

FIG. 5 is an example multi-panel antenna array;

FIG. 6 is an example of oversampled DFT beams;

FIG. 7 is a block diagram illustrating an example wireless network,according to a particular embodiment;

FIG. 8 is an example of a non-coherent multi-panel/TRP transmission,according to some embodiments;

FIG. 9A is flow diagram illustrating an example method in a userequipment, according to some embodiments;

FIG. 9B is flow diagram illustrating an example method in a networknode, according to some embodiments;

FIG. 10A is a block diagram illustrating an example embodiment of awireless device;

FIG. 10B is a block diagram illustrating example components of awireless device;

FIG. 11A is a block diagram illustrating an example embodiment of anetwork node;

FIG. 11B is a block diagram illustrating example components of a networknode; and

FIG. 12 is a block diagram illustrating configuration of an example CSIframework for NC-JT.

DETAILED DESCRIPTION

The next generation mobile wireless communication system (5G) or newradio (NR) supports a wide range of antenna setups, deployments, and usecases compared to long term evolution (LTE), such as multi-panel andmulti-transmission reception point (TRP) operation in both the uplinkand downlink. A particular problem is how to efficiently support suchflexibility, yet avoid specifying separate channel state information(CSI) feedback format for each use case. In the downlink, a userequipment (UE) may be served by a transmit/receive point (TRP) equippedwith multiple antenna panels, or it may be served by multiple TRPs, eachequipped with one or more antenna panels. The panels may use differentport layouts and would thus benefit from being associated with differentprecoder codebooks. If a single CSI-RS resource comprises the ports frommultiple panels, a variety of different precoder codebooks correspondingto different combinations of panel sizes may have to be specified, whichis infeasible.

Particular embodiments obviate the problems described above and includea UE configured to measure on multiple CSI-RS resources, where eachresource corresponds to a separate transmit/receive point (TRP) orantenna panel. The UE may select a number of channel state informationreference signal (CSI-RS) resources to participate in a non-coherentjoint transmission, as well as a preferred precoder matrix for eachCSI-RS resource, on the basis that layers corresponding to differentCSI-RS resources mutually interfere.

A UE may jointly select a preferred precoder matrix and rank for eachTRP/panel and calculate an appropriate channel quality indicator (CQI)under a non-coherent joint transmission (NC-JT) assumption. Byassociating each CSI-RS resource with a separate precoder codebook,particular embodiments may support a variety of different port layoutsand antenna deployments.

The following description sets forth numerous specific details. It isunderstood, however, that embodiments may be practiced without thesespecific details. In other instances, well-known circuits, structuresand techniques have not been shown in detail in order not to obscure theunderstanding of this description. Those of ordinary skill in the art,with the included descriptions, will be able to implement appropriatefunctionality without undue experimentation.

References in the specification to “one embodiment,” “an embodiment,”“an example embodiment,” etc., indicate that the embodiment describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to implement such feature, structure, orcharacteristic in connection with other embodiments, whether or notexplicitly described.

Particular embodiments are described with reference to FIGS. 7-11B ofthe drawings, like numerals being used for like and corresponding partsof the various drawings. LTE and NR are used throughout this disclosureas an example cellular system, but the ideas presented herein may applyto other wireless communication systems as well.

FIG. 7 is a block diagram illustrating an example wireless network,according to a particular embodiment. Wireless network 100 includes oneor more wireless devices 110 (such as mobile phones, smart phones,laptop computers, tablet computers, MTC devices, V2X devices, or anyother devices that can provide wireless communication) and a pluralityof network nodes 120 (such as base stations or eNodeBs). Wireless device110 may also be referred to as a UE. Network node 120 serves coveragearea 115 (also referred to as cell 115).

In general, wireless devices 110 that are within coverage of networknode 120 (e.g., within cell 115 served by network node 120) communicatewith network node 120 by transmitting and receiving wireless signals130. For example, wireless devices 110 and network node 120 maycommunicate wireless signals 130 containing voice traffic, data traffic,and/or control signals.

A network node 120 communicating voice traffic, data traffic, and/orcontrol signals to wireless device 110 may be referred to as a servingnetwork node 120 for the wireless device 110. Communication betweenwireless device 110 and network node 120 may be referred to as cellularcommunication. Wireless signals 130 may include both downlinktransmissions (from network node 120 to wireless devices 110) and uplinktransmissions (from wireless devices 110 to network node 120). In LTE,the interface for communicating wireless signals between network node120 and wireless device 110 may be referred to as a Uu interface.

Each network node 120 may have a single transmitter or multipletransmitters for transmitting signals 130 to wireless devices 110. Insome embodiments, network node 120 may comprise a multi-inputmulti-output (MIMO) system. For example, network node may include amulti-panel or multi-TRP antenna system. Similarly, each wireless device110 may have a single receiver or multiple receivers for receivingsignals 130 from network nodes 120 or other wireless devices 110.

Wireless devices 110 may communicate with each other (i.e., D2Doperation) by transmitting and receiving wireless signals 140. Forexample, wireless device 110 a may communicate with wireless device 110b using wireless signal 140. Wireless signal 140 may also be referred toas sidelink 140. Communication between two wireless devices 110 may bereferred to as D2D communication or sidelink communication. In LTE, theinterface for communicating wireless signal 140 between wireless devices110 may be referred to as a PC5 interface.

Wireless signals 130 and 140 may be transmitted on time-frequencyresources. The time-frequency resources may be partitioned into radioframes, subframes, slots, and/or mini-slots. Data may be scheduled fortransmission based on the partitions. For example, data transmissionsmay be scheduled based on subframe, slot, or mini-slot.

Wireless device 110, network node 120, or any other component of network100 that transmits wireless signals may be referred to as a wirelesstransmitter. Wireless device 110, network node 120, or any othercomponent of network 100 that receives wireless signals may be referredto as a wireless receiver.

Wireless signals 130 may include reference signals, such as CSI-RS.Network node 120 may transmit one or more CSI-RS signal on a set oftime-frequency resource elements (REs) associated with an antenna port.Wireless device 110 may be configured to receive and measure the CSI-RS.Wireless device 110 may transmit CSI reports to one or more networknodes 120.

For example, network node 120 may transmit, to wireless device 110, afirst CSI-RS in a first CSI-RS resource of a set of at least two CSI-RSresources from a first antenna subset and a second CSI-RS in a secondCSI-RS resource in the set of at least two CSI-RS resources from asecond antenna subset. An antenna subset is one or more antenna elementscomprised on at least a single antenna panel comprising an antennaarray. A single antenna panel may be associated with a single TRP. Amulti-panel array may be associated with a single TRP, wherein anantenna subset may comprise one or more antenna panels.

In particular embodiments, wireless device 110 may configured (e.g.,preconfigured, or dynamically configured via signaling from anothernetwork element, such as network node 120) with two or more CSI-RSresources in a CSI-RS resource set. Wireless device 110 may select atleast two CSI-RS resources from the CSI-RS resource set, determine apreferred precoder matrix for each of the selected CSI-RS resources, andtransmit a CSI report to network node 120 indicating each of theselected CSI-RS resources and the determined preferred precoder matrixfor each of the selected CSI-RS resources. Wireless device 110 maycalculate a channel estimate for each of the selected CSI-RS resourcesand determine a CQI corresponding to a hypothetical transmission from aplurality of effective channels. The effective channels depend on thepreferred precoder matrices and channel estimate for each of theselected CSI-RS resources. Layers transmitted through the effectivechannels mutually interfere. The CSI report further indicates thedetermined CQI for each of the selected CSI-RS resources.

Network node 120 may receive, from wireless device 110, a CSI reportcomprising a first preferred precoder matrix associated with the firstCSI-RS resource and a second preferred precoder matrix associated withthe second CSI-RS resource. The CSI report may further comprise a CQIassociated with each of the first and second preferred precodermatrices. The antenna subsets belong to different transmission points orbelong to different antenna panels of the same transmission point. Inparticular embodiments, wireless device 110 may receive CSI-RS andreport CSI according to any of the examples and embodiments describedwith respect to FIGS. 8-11B.

In wireless network 100, each network node 120 may use any suitableradio access technology, such as long term evolution (LTE), 5G NR,LTE-Advanced, UMTS, HSPA, GSM, cdma2000, NR, WiMax, WiFi, and/or othersuitable radio access technology. Wireless network 100 may include anysuitable combination of one or more radio access technologies. Forpurposes of example, various embodiments may be described within thecontext of certain radio access technologies. However, the scope of thedisclosure is not limited to the examples and other embodiments coulduse different radio access technologies.

As described above, embodiments of a wireless network may include one ormore wireless devices and one or more different types of radio networknodes capable of communicating with the wireless devices. The networkmay also include any additional elements suitable to supportcommunication between wireless devices or between a wireless device andanother communication device (such as a landline telephone). A wirelessdevice may include any suitable combination of hardware and/or software.For example, in particular embodiments, a wireless device, such aswireless device 110, may include the components described with respectto FIG. 10A below. Similarly, a network node may include any suitablecombination of hardware and/or software. For example, in particularembodiments, a network node, such as network node 120, may include thecomponents described with respect to FIG. 11A below.

Particular embodiments include CSI feedback to support non-coherentjoint transmission (NC-JT) from multiple panels or multiple transmissionpoints. With NC-JT, separate layers are transmitted from each antennapanel or transmission/reception point (TRP) to increase the transmissionrank at the UE and correspondingly increase the achievable data rate. Aparticular benefit of non-coherent JT is to facilitate higher ranktransmission in the case where the UE is rank-constrained, e.g. by beingline-of-sight (LOS) to the serving transmission point or if the servingtransmission point only supports a few layers. By transmittingadditional layers from a non-serving transmission point, the UE's peakrate can be increased.

For NC-JT to be beneficial, accurate link adaptation is required asthere can be significant inter-layer interference between thetransmissions from the multiple TRPs or panels. Furthermore, it isbeneficial to select the transmission rank and precoding of theparticipating TRPs jointly so that an optimal transmission setting maybe used.

In some embodiments, a UE is configured to measure on a number ofnon-zero power (NZP) CSI-RS resources, where each CSI-RS resourcecomprises a number of CSI-RS antenna ports. Each CSI-RS resource may beassociated with a different TRP or antenna panel. In some embodiments,the CSI-RS carried in a CSI-RS resource may be transmitted fromdifferent antenna subsets of the antenna array of a TRP in anon-precoded fashion, while in other embodiments, the CSI-RS aretransmitted from all antennas of the antenna array of a TRP in abeamformed fashion.

Some embodiments use the NR CSI framework and configure the UE with aCSI Report Setting that is linked with one Resource Setting for channelmeasurement. The Resource Setting may comprise a resource set withmultiple CSI-RS resources, where each CSI-RS resource corresponds to aseparate TRP.

In other embodiments, the UE is configured with a CSI Report Settinglinked with several Resource Settings for channel measurement where eachResource Setting is associated with a separate TRP. Each ResourceSetting may comprise a set of multiple CSI-RS resources, or they maycomprise a set of a single CSI-RS resource.

Regardless of how the CSI-RS resources are configured in the CSIframework and whether they belong to the same Resource Setting, the UEmay in some embodiments, select a number of CSI-RS resourcescorresponding to a number of TRPs to indicate that a NC-JT from the TRPsis desired. In some embodiments, the selection is made with one or moreCSI-RS Resource Indicator (CRI).

For example, the UE may report a “Number of Resources Indicator” (NRI),which indicates how many resources are selected along with a set of theselected CRIs: {CRI₁, . . . , CRI_(NRI)}. Alternatively, the CRIselection may be reported using a bitmap with each bit corresponding toa CSI-RS resource. Setting the corresponding bit to one indicates thatthe resource is selected.

In other embodiments, each TRP transmits CSI-RS in several CSI-RSresources (if for example each TRP corresponds to a Resource Settingwhich in turn comprises one or more set of CSI-RS resources)corresponding to different beams in a beam sweep transmitted from a TRP.The UE may select both which TRPs it desires to participate in the NC-JTas well as which CSI-RS resource should be used per TRP. In some suchembodiments, the TRP selection is made with one or more separateResource Setting Indicator (RSI), while the selection of a CSI-RSresource per TRP (i.e., within the set of CSI-RS resources within aResource Setting) is made with a CRI.

In some embodiments, the TRP selection is made with a single hypothesisindicator (HI). In some embodiments, the UE is configured with a set ofpossible hypotheses for dynamic (transmission) point selection and NC-JTin the CSI Report Setting, for example according to Table 1 below wherea ‘1’ indicates that a TRP is transmitting and a ‘0’ indicates theopposite. In the example, the selection of HI indicates which TRPs theUE desires to participate in the transmission.

In some embodiments, the HI selection corresponds to selection of one ormore Resource Setting, where additionally a CSI-RS resource within eachResource Setting may be selected. In other embodiments, the HI selectioncorresponds to selection of one or more CSI-RS resources and isfunctionally equivalent to indicating multiple CRIs. However, bypreconfiguring a number of hypotheses, the signaling overhead may bereduced and additionally the number of possible hypotheses a UE mayselect can be constrained. For example, the network may only supportNC-JT from a smaller number of TRPs than what the UE is configured tomeasure on (for instance, maximum two TRPs out of the three in theexample below, in that case, the network may not configure HI=7). Inanother embodiment, the UE is configured to report multiple CSIscorresponding to multiple hypotheses in the same CSI report.

TABLE 1 Example of DPS and NC-JT hypotheses Hypothesis Indicator (HI)TRP #1 TRP #2 TRP #3 1 1 0 0 2 0 1 0 3 0 0 1 4 1 1 0 5 0 1 1 6 1 0 1 7 11 1

In other embodiments, the UE is configured to report CSI for a certainNC-JT hypothesis and on the basis that the configured Resource Settingsor NZP CSI-RS resources shall participate in the NC-JT.

Each CSI-RS resource may be associated with a separate quasi-colocation(QCL) assumption. For example, it may be assumed that antenna portswithin a CSI-RS resource are transmitted from a single TRP, and thus arequasi-colocated, but ports in different CSI-RS resources cannot beassumed to be transmitted from a single TRP, and therefore cannot beassumed quasi-colocated.

Each CSI-RS resource is associated with a precoder codebook. In someembodiments, the precoder codebook is a parametrized codebook thatdepends of the port layout (N₁, N₂) of the CSI-RS resource, where thenumber of ports in the CSI-RS resource is P=2N₁N₂. In some embodiments,the port layout is associated with the CSI-RS resource in the ResourceSetting and only a codebook Type (such as NR Type I or Type II) isidentified in the Report Setting. In some embodiments, a codebook (whichmay be a function of the port layout) is identified for each linkedResource Setting in the CSI Report Setting.

For each of the selected CSI-RS resources, the UE calculates a preferredprecoder matrix from the associated codebook, under the assumption thattransmission occurs from all of the CSI-RS resources simultaneously. Theresulting rank the UE shall select for the hypothetical transmission isthus the sum of the per-resource ranks: ν_(TOT)=Σ_(k×1) ^(K)ν_(k), whereν_(k) is the rank of the precoder hypothesis for the selected CSI-RSresource k and K is the number of selected resources. The UE makes theprecoder selection, on the basis that the layers corresponding todifferent CSI-RS resources mutually interfere.

For example, if W_(k) is the desired precoder matrix of rank ν_(k) forCSI-RS resource k∈{1 . . . K} and H_(k) is the channel estimate of theCSI-RS ports of resource k, the following effective channel is used forthe hypothetical PDSCH transmission when determining PMI and CQI:

$H_{eff} = {{\left\lbrack {{H_{1}...}\mspace{14mu} H_{K}} \right\rbrack\begin{bmatrix}W_{1} & \cdots & 0 \\\vdots & \ddots & \vdots \\0 & \cdots & W_{K}\end{bmatrix}} = {\left\lbrack {{{H_{1}W_{1}}...}\mspace{14mu} H_{K}W_{K}} \right\rbrack.}}$

In specification language, this may be described as what correspondencebetween CSI-RS ports and DMRS ports shall be assumed by the UE for thehypothetical transmission when determining CQI. Assuming LTE portnumbering such that DMRS antenna ports are numbered between 7-14 andCSI-RS antenna ports are numbered between 15-31, the specification textmay look as follows:

PDSCH signals on antenna ports {p_(k), p_(k+1)}, where p_(k)=7+Σ_(l=1)^(k−1)v_(l) would result in signals equivalent to corresponding symbolstransmitted on antenna ports {15, . . . , 14+P_(k)} corresponding toCSI-RS resource k in a set of K CSI-RS resources, as given by[y ⁽¹⁵⁾ . . . y ^((14+P) ^(k) ⁾]^(T) =W _(k)[x ^((p) ^(k) ⁾ . . . x^((p) ^(k+1) ⁾]^(T)where x⁽⁷⁾, . . . , x^((6+v) ^(TOT) ⁾ with v_(TOT)×Σ_(k=1) ^(K)v_(k) isa vector of symbols, where each symbol corresponds to a layer to betransmitted to the UE and contains all layers from all TRPs transmittingthe PDSCH to the UE. In some embodiments, the vector x may correspond tothe vector x from the layer mapping in Subclause 6.3.3.2 of 3GPPtechnical specification 36.211, e.g. a set of layers being a subset ofall layers from all TRPs.

In some embodiments, indicating the preferred precoder matrix maycomprise determining a PMI and RI for each selected CSI-RS resource. Ifa multi-stage codebook and frequency-selective precoding is used,several precoder matrices may be indicated with multiple PMI, forinstance, W_(k)(f)=W_(1,k) W_(2,k) (f).

The UE will then calculate a Channel Quality Indicator (CQI)corresponding to the preferred precoders and under the assumption thatlayers corresponding to the selected CSI-RS resources mutuallyinterfere. Such mutual interference may be calculated based on Equation1 using y=H_(eff)x, where y is a hypothesized received signal of a PDSCHcomprising the mutual interference among the MIMO layers in xtransmitted on the TRPs hypothesized to be transmitting the PDSCH to theUE.

In NR, NC-JT transmission from one or more TRPs can be achieved in twoways: either a single PDSCH is transmitted, where the layers within thePDSCH are transmitted from different TRPs, or multiple PDSCHs aretransmitted each from a separate TRP. As a codeword-to-layer mapping isapplied within a PDSCH, different number of layers per codeword, andcorrespondingly per CQI (where one codeword maps to one CQI), may beused for the same number of layers, depending on which way is used.Thus, in an embodiment, the codeword-to-layer mapping as well as thenumber of CQIs to calculate is indicated in the Report Setting, so thatthe reported CQI correspond to the subsequent PDSCH transmission.

In another embodiment, a non-ideal backhaul link between the TRPsparticipating in the NC-JT is assumed. In this case, a separate CSIreport may be transmitted to each TRP, comprising only the PMI(s), RIand CQI(s) corresponding to that TRP. In other embodiments, a single CSIreport is transmitted comprising the PMIs, RIs and CQIs corresponding tothe different resources.

FIG. 8 illustrates an example of non-coherent multi-panel/TRPtransmission, according to some embodiments. The UE is configured withfive 2-port CSI-RS resources and selects two resources (resources #1 and#5).

The examples and embodiments described above may be generalized by theflowcharts in FIGS. 9A and 9B.

FIG. 9A is flow diagram illustrating an example method in a userequipment, according to some embodiments. In particular embodiments, oneor more steps of FIG. 9A may be performed by wireless device 110 ofnetwork 100 described with respect to FIG. 7.

The method may begin at step 910, where the UE obtains a CSI reportconfiguration. For example, in some embodiments wireless device 110 mayreceive a report configuration from network node 120. The CSI reportconfiguration may include any suitable configuration information forinstructing the UE how to measure CSI-RS resources and how to report themeasurements to the network node.

In some embodiments, the CSI report configuration may include possiblehypotheses for combinations of one or more CSI-RS resources. Forexample, the report configuration may include a Hypothesis Indicator(HI) as described above with respect to Table 1. In another example, theUE network may transmit CSI-RS in four CSI-RS resources. The CSI reportconfiguration may include an indicator with values 1, 2, or 3. If theindicator value is 2, for example, then the UE reports CSI information(e.g., PMI/CQI/RI) for two CSI-RS resources.

In some embodiments, the UE may be preconfigured with CSI reportconfiguration information and obtaining the CSI report configuration maycomprise reading the CSI report configuration information from memory orstorage.

In some embodiments, the configuration information may include anindication of a codeword-to-layer mapping for use when determining theCQI.

At step 912, the UE may receive an instruction to measure a set ofCSI-RS resources. For example, wireless device 110 may receive aninstruction from network node 120 to measure CSI-RS on a set of N CSI-RSresources. In some embodiments, each CSI-RS resource may carry one ormore CSI-RS.

In some embodiments, wireless device 110 may be preconfigured with a setof CSI-RS resources and a schedule for measuring the CSI-RS resources.In some embodiments, wireless device 110 may be preconfigured with adefault configuration and may receive instructions to override thedefault configuration.

At step 914, the UE may calculate a channel estimate for each of theCSI-RS resources in the set. For example, wireless device 110 maymeasure and determine a channel estimate for each CSI-RS.

At step 916, the UE selects at least two CSI-RS resources from the setof CSI-RS resources. For example, wireless device 110 may select asubset K of CSI-RS from a set of N CSI-RS. K is less than or equal to N(e.g., in some embodiments the subset may include the entire set) andgreater than or equal to 2. Wireless device 100 may select the subsetaccording to any of the examples or embodiments described above. Forexample, selecting the subset of CSI-RS resources may comprise selectinga plurality of CSI-RS resource indicators (CRIs), or selecting aplurality of Resource Settings. The selection may be determined based ona hypothesis indicator.

At step 918, the UE determines, for each of the selected CSI-RSresources, one or more preferred precoder matrices. For example,wireless device 110 may determine one or more preferred precodermatrices according to any of the examples or embodiments describedabove.

As one example, the determined precoder may be a single widebandprecoder for the entire system bandwidth. As another example, the UE maydetermine multiple precoders for a CSI-resource. The multiple precodersmay each comprise frequency selective (or subband) precoders.

At step 920, the UE may determine a CQI corresponding to a hypotheticaltransmission from a number of effective channels. The effective channelsdepend on the preferred precoder matrices and channel estimate for eachof the CSI-RS resources in the subset. Layers transmitted through theeffective channels mutually interfere. For example, wireless device 110may determine a CQI based on the preferred precoder matrices and channelestimate for the subset of CSI-RS resources according to any of theexamples or embodiments described above.

At step 922, the UE transmits a CSI report to one or more network nodes.Each CSI report may comprise one or more preferred precoder matrices andthe one or more CQI. For example, wireless device 110 may transmit a CSIreport to network node 120 according to any of the embodiments andexamples described above. In some embodiments, the CSI report mayinclude multiple messages (e.g., one message per CSI-RS resource). Themultiple message may be sent to one network element, or the message maybe sent to different network elements, each one responsible for one ormore of the TRPs or antenna panels.

Modifications, additions, or omissions may be made to method 900 of FIG.9A. Additionally, one or more steps in the method of FIG. 9A may beperformed in parallel or in any suitable order. The steps may berepeated over time as necessary.

FIG. 9B is flow diagram illustrating an example method in a networknode, according to some embodiments. In particular embodiments, one ormore steps of FIG. 9B may be performed by network node 120 of network100 described with respect to FIG. 7.

The method may begin at step 952, where the network node transmits a CSIreport configuration to user equipment. For example, network node 120may transmit a CSI report configuration to wireless device 110. The CSIreport configuration is described in more detail above with respect tostep 910 of FIG. 9A.

At step 954, the network node transmits a set of CSI-RS resources from aplurality of transmission points to a wireless device, for example, afirst CSI-RS in a first CSI-RS resource of a set of at least two CSI-RSresources from a first antenna subset and a second CSI-RS in a secondCSI-RS resource in the set of at least two CSI-RS resources from asecond antenna subset. For example, network node 120 may transmit a setof CSI-RS to wireless device 110 using three transmission points. Atransmission point may refer to a TRP, an antenna panel, etc. Networknode 120 may transmit a first CSI-RS in a first CRI-RS resource from thefirst TRP, a second CSI-RS in a second CSI-RS resource from the secondTRP, and so on. Since the first and second antenna subsets may comprisea first and second set of spatially multiplexed layers, respectively,wherein the first and second set of layers are different, each of thetransmissions may be joint (i.e., at the same time) and non-coherent(i.e., may interfere with each other).

At step 956, the network node receives, from the wireless device, a CSIreport comprising a first preferred precoder matrix associated with thefirst CSI-RS resource and a second preferred precoder matrix associatedwith the second CSI-RS resource. The CSI report may further comprise aCQI associated with each of the first and second preferred precodermatrices. For example, network node 120 may receive a CSI report fromwireless device 110. Wireless device 110 may determine the CSI reportaccording to any of the examples and embodiments described above.

Modifications, additions, or omissions may be made to method 950 of FIG.9B. Additionally, one or more steps in the method of FIG. 9B may beperformed in parallel or in any suitable order. The steps may berepeated over time as necessary.

FIG. 10A is a block diagram illustrating an example embodiment of awireless device. The wireless device is an example of the wirelessdevices 110 illustrated in FIG. 7. In particular embodiments, thewireless device is capable of receiving an instruction to measure a setof CSI-RS resources. Each CSI-RS resource of the set of CSI resourcescarries a CSI-RS. The wireless device may be further capable ofcalculating a channel estimate for each of the CSI-RS resources in theset; determining, for each of the CSI-RS resources in the set, one ormore preferred precoder matrices; determining CQI corresponding to ahypothetical transmission from a number of effective channels, where theeffective channels depend on the preferred precoder matrices and channelestimate for each of the CSI-RS resources, and layers transmittedthrough the effective channels mutually interfere; and transmitting oneor more CSI reports to one or more network nodes. Each CSI report maycomprise a plurality of one or more preferred precoder matrices and theone or more CQI.

Particular examples of a wireless device include a mobile phone, a smartphone, a PDA (Personal Digital Assistant), a portable computer (e.g.,laptop, tablet), a sensor, a modem, a machine type (MTC) device/machineto machine (M2M) device, laptop embedded equipment (LEE), laptop mountedequipment (LME), USB dongles, a device-to-device capable device, avehicle-to-vehicle device, or any other device that can provide wirelesscommunication. The wireless device includes transceiver 1010, processingcircuitry 1020, memory 1030, and power source 1040. In some embodiments,transceiver 1010 facilitates transmitting wireless signals to andreceiving wireless signals from wireless network node 120 (e.g., via anantenna), processing circuitry 1020 executes instructions to providesome or all of the functionality described herein as provided by thewireless device, and memory 1030 stores the instructions executed byprocessing circuitry 1020. Power source 1040 supplies electrical powerto one or more of the components of wireless device 110, such astransceiver 1010, processing circuitry 1020, and/or memory 1030.

Processing circuitry 1020 includes any suitable combination of hardwareand software implemented in one or more integrated circuits or modulesto execute instructions and manipulate data to perform some or all ofthe described functions of the wireless device. In some embodiments,processing circuitry 1020 may include, for example, one or morecomputers, one more programmable logic devices, one or more centralprocessing units (CPUs), one or more microprocessors, one or moreapplications, and/or other logic, and/or any suitable combination of thepreceding. Processing circuitry 1020 may include analog and/or digitalcircuitry configured to perform some or all of the described functionsof wireless device 110. For example, processing circuitry 1020 mayinclude resistors, capacitors, inductors, transistors, diodes, and/orany other suitable circuit components.

Memory 1030 is generally operable to store computer executable code anddata. Examples of memory 1030 include computer memory (e.g., RandomAccess Memory (RAM) or Read Only Memory (ROM)), mass storage media(e.g., a hard disk), removable storage media (e.g., a Compact Disk (CD)or a Digital Video Disk (DVD)), and/or or any other volatile ornon-volatile, non-transitory computer-readable and/orcomputer-executable memory devices that store information.

Power source 1040 is generally operable to supply electrical power tothe components of wireless device 110. Power source 1040 may include anysuitable type of battery, such as lithium-ion, lithium-air, lithiumpolymer, nickel cadmium, nickel metal hydride, or any other suitabletype of battery for supplying power to a wireless device.

Other embodiments of the wireless device may include additionalcomponents (beyond those shown in FIG. 10A) responsible for providingcertain aspects of the wireless device's functionality, including any ofthe functionality described above and/or any additional functionality(including any functionality necessary to support the solution describedabove).

FIG. 10B is a block diagram illustrating example components of awireless device 110. The components may include receiving module 1050,determining module 1052, and transmitting module 1054.

Receiving module 1050 may perform the receiving functions of wirelessdevice 110. For example, receiving module 1050 may receive a CSIconfiguration and/or an instruction to measure a set of CSI-RS resourcesaccording to any of the examples and embodiments described above (e.g.,steps 910 and 912 of FIG. 9A). In certain embodiments, receiving module1050 may include or be included in processing circuitry 1020. Inparticular embodiments, receiving module 1050 may communicate withdetermining module 1052 and transmitting module 1054.

Determining module 1052 may perform the determining functions ofwireless device 110. For example, determining module 1052 may select twoor more CSI-RS resources from a set of CSI-RS resources, determine aprecoder matrix for each of the selected CSI-RS resources, determine achannel estimate for each of the selected CSI-RS resources, and/ordetermine a CQI corresponding to a hypothetical transmission from anumber of effective channels, where the effective channels depend on thedetermined precoder matrices and channel estimate for each of theselected CSI-RS resources, and layers transmitted through the effectivechannels mutually interfere (e.g., steps 916-920 of FIG. 9A). In certainembodiments, determining module 1052 may include or be included inprocessing circuitry 1020. In particular embodiments, determining module1052 may communicate with receiving module 1050 and transmitting module1054.

Transmitting module 1054 may perform the transmitting functions ofwireless device 110. For example, transmitting module 1054 may transmitone or more CSI reports to one or more network nodes according to any ofthe examples and embodiments described above (e.g., step 922 of FIG.9A). In certain embodiments, transmitting module 1054 may include or beincluded in processing circuitry 1020. In particular embodiments,transmitting module 1054 may communicate with receiving module 1050 anddetermining module 1052.

FIG. 11A is a block diagram illustrating an example embodiment of anetwork node. The network node is an example of the network node 120illustrated in FIG. 7. In particular embodiments, the network node iscapable of transmitting two or more CSI-RS to a wireless device andreceiving a CSI report from a wireless device with different channelstate information for two or more CSI-RS.

Network node 120 can be an eNodeB, a nodeB, gNB, a base station, awireless access point (e.g., a Wi-Fi access point), a low power node, abase transceiver station (BTS), a transmission point or node, a remoteRF unit (RRU), a remote radio head (RRH), or other radio access node.The network node includes at least one transceiver 1110, processor orprocessing circuitry 1120, at least one memory 1130, and at least onenetwork interface 1140. Transceiver 1110 facilitates transmittingwireless signals to and receiving wireless signals from a wirelessdevice, such as wireless devices 110 (e.g., via an antenna); processingcircuitry 1120 executes instructions to provide some or all of thefunctionality described above as being provided by a network node 120;memory 1130 stores the instructions executed by processing circuitry1120; and network interface 1140 communicates signals to backend networkcomponents, such as a gateway, switch, router, Internet, Public SwitchedTelephone Network (PSTN), controller, and/or other network nodes 120.Processing circuitry 1120 and memory 1130 can be of the same types asdescribed with respect to processing circuitry 1020 and memory 1030 ofFIG. 10A above.

In some embodiments, network interface 1140 is communicatively coupledto processing circuitry 1120 and refers to any suitable device operableto receive input for network node 120, send output from network node120, perform suitable processing of the input or output or both,communicate to other devices, or any combination of the preceding.Network interface 1140 includes appropriate hardware (e.g., port, modem,network interface card, etc.) and software, including protocolconversion and data processing capabilities, to communicate through anetwork.

Other embodiments of network node 120 include additional components(beyond those shown in FIG. 11A) responsible for providing certainaspects of the network node's functionality, including any of thefunctionality described above and/or any additional functionality(including any functionality necessary to support the solution describedabove). The various different types of network nodes may includecomponents having the same physical hardware but configured (e.g., viaprogramming) to support different radio access technologies, or mayrepresent partly or entirely different physical components.

FIG. 11B is a block diagram illustrating example components of a networknode 120. The components may include transmitting module 1150 andreceiving module 1152.

Transmitting module 1150 may perform the transmitting functions ofnetwork node 120. For example, transmitting module 1150 may transmit aset of CSI-RS resources to a wireless device according to any of theexamples and embodiments described above (e.g., step 954 of FIG. 9A).Transmitting module 1150 may transmit a CSI report configuration to awireless device (e.g., step 952 of FIG. 9A). In certain embodiments,transmitting module 1150 may include or be included in processingcircuitry 1120. In particular embodiments, transmitting module 1150 maycommunicate with receiving module 1152.

Receiving module 1152 may perform the receiving functions of networknode 120. For example, receiving module 1152 may receive a CSI reportaccording to any of the examples and embodiments described above (e.g.,step 956 of FIG. 9A). In certain embodiments, receiving module 1152 mayinclude or be included in processing circuitry 1120. In particularembodiments, receiving module 1152 may communicate with transmittingmodule 1150.

Modifications, additions, or omissions may be made to the systems andapparatuses disclosed herein without departing from the scope of theinvention. The components of the systems and apparatuses may beintegrated or separated. Moreover, the operations of the systems andapparatuses may be performed by more, fewer, or other components.Additionally, operations of the systems and apparatuses may be performedusing any suitable logic comprising software, hardware, and/or otherlogic. As used in this document, “each” refers to each member of a setor each member of a subset of a set.

Modifications, additions, or omissions may be made to the methodsdisclosed herein without departing from the scope of the invention. Themethods may include more, fewer, or other steps. Additionally, steps maybe performed in any suitable order.

Although this disclosure has been described in terms of certainembodiments, alterations and permutations of the embodiments will beapparent to those skilled in the art. Accordingly, the above descriptionof the embodiments does not constrain this disclosure. Other changes,substitutions, and alterations are possible without departing from thespirit and scope of this disclosure, as defined by the claims below.

The examples below provide a non-limiting example of how certain aspectsof the embodiments may be implemented within the framework of a specificcommunication standard. In particular, the examples provide anon-limiting example of how particular embodiments could be implementedwithin the framework of a 3GPP TSG RAN standard. The changes are merelyintended to illustrate how certain aspects of the embodiments could beimplemented in a particular standard. However, the embodiments couldalso be implemented in other suitable manners, both in the 3GPPSpecification and in other specifications or standards.

For example, a standard may include the following attributes related toa CSI framework. Regarding RS and interference measurement setting, aspecification may rename ‘RS setting” as “Resource setting,” whichcomprises configuration for a signal used for channel and/orinterference measurement.

Regarding other terminology, a UE can be configured with N≥1 CSIreporting settings, M≥1 Resource settings, and 1 CSI measurementsetting, where the CSI measurement setting includes L≥1 links. Each ofthe L links corresponds to a CSI reporting setting and a Resourcesetting.

The following configuration parameters may be signaled via radioresource control (RRC) at least for CSI acquisition. N, M, and L may beindicated either implicitly or explicitly. In each CSI reportingsetting, at least reported CSI parameter(s), CSI Type (I or II) ifreported, codebook configuration including codebook subset restriction,time-domain behavior, frequency granularity for CQI and PMI, measurementrestriction configurations may be signaled. In each Resource setting, aconfiguration of S≥1 CSI-RS resource set(s) may be signaled. Each setmay correspond to different selections from a pool of all CSI-RSresources configured to the UE. A configuration of Ks≥1 CSI-RS resourcesfor each set s, including at least mapping to REs, the number of ports,time-domain behavior, etc., may be signaled. In each of the L links inCSI measurement setting, CSI reporting setting indication, Resourcesetting indication, and/or quantity to be measured (either channel orinterference) may be signaled. One CSI reporting setting can be linkedwith one or multiple Resource settings. Multiple CSI reporting settingscan be linked with the same Resource setting.

The following may be dynamically selected by L1 or L2 signaling, ifapplicable: (a) one or multiple CSI reporting settings within the CSImeasurement setting; (b) one or multiple CSI-RS resource sets selectedfrom at least one Resource setting; and (c) one or multiple CSI-RSresources selected from at least one CSI-RS resource set.

Regarding NR reception, a single NR-PDCCH may schedule a single NR-PDSCHwhere separate layers are transmitted from separate TRPs. MultipleNR-PDCCHs each may schedule a respective NR-PDSCH where each NR-PDSCH istransmitted from a separate TRP. The case where a single NR-PDCCHschedules a single NR-PDSCH where each layer is transmitted from allTRPs jointly can be done in a specification-transparent manner. CSIfeedback details for the above case can be specified separately.

With non-coherent joint transmission (NC-JT), separate layers aretransmitted from each antenna panel or transmission point (TRP) toincrease the transmission rank at the UE and correspondingly increasethe achievable data rate. The primary benefit of non-coherent JT is tofacilitate higher rank transmission where the UE is rank-constrained(e.g., by being line-of-sight (LOS) to the serving transmission point orif the serving transmission point only supports a few layers). Bytransmitting additional layers from a non-serving transmission point,the UE's peak rate can be increased. For NC-JT to be beneficial though,accurate link adaptation is required as there can be significantinter-layer interference between the transmissions from the multipleTRPs or panels. Furthermore, it is beneficial to select the transmissionrank and precoding of the participating TRPs jointly so that an optimaltransmission setting can be used.

NC-JT can be supported using the existing CSI framework in NR. Forexample, a UE could be configured with two CSI Report Settings, eachcorresponding to a separate TRP, and to feed back a PMI/RI/CQI reportcorresponding to each TRP corresponding to a single-TRP hypothesis. Thereported PMIs can be used directly to precode the transmission from eachTRP. However, there are several issues with using thestandard-transparent approach: (a) the RIs will likely be chosen tooaggressively because they correspond to single-TRP hypothesis and arenot selected jointly corresponding to NC-JT hypothesis; (b) the CQIswill be too optimistic because inter-TRP interference is not taken intoaccount; and (c) because PMIs for each TRP are determined independently,PMIs that cause large mutual interference could be selected by the UE.

Thus, a CSI Report Setting that supports NC-JT in the NR CSI frameworkis beneficial. For optimal performance, the UE may dynamically selectbetween single-TRP (i.e., DPS) or NC-JT transmission from multiple TRPs.The selection may be fed back in a single CSI report. Even if the UEindicates that it prefers NC-JT transmission, however, the UE cannot becertain that the network can accommodate NC-JT transmission andsingle-TRP transmission may be applied instead. Thus, the network needsCSI available for both NC-JT and single-TRP hypothesis, which means thatseveral CSI Report Settings may be configured for the UE.

Particular 3GPP working groups considered the following options for CSIenhancements for NC-JT CSI feedback. Option 1 uses a single CSI processwith K>1 CSI-RS resources. Channel measurement and inter-TP interferencemeasurement can be flexibly configured based on the selection of these KCSI-RS resource for different hypothesis.

Option 2 uses a single CSI process with enhanced codebook and anaggregated CSI resource from multiple CSI-RS resources. CSI-RS resourcesfrom multiple TPs are aggregated to form one CSI-RS resource. Anenhanced codebook with the codeword structure considering non-coherentjoint transmission can be applied to the aggregated channel measuredfrom the aggregated CSI-RS. An example of the codeword structureconsidering two-TP joint transmission is

${W = \begin{bmatrix}W_{a} & 0 \\0 & W_{b}\end{bmatrix}},$where W_(a) and W_(b) are the precoding matrices applied to the two TPsrespectively.

Option 3 uses multiple CSI processes with dependency among CSIprocesses. For the multiple CSI processes mechanism, dependency amongthe CSI-processes can be considered. In this way, different interferenceassumption for each TP is indicated. More specially, the calculated CSIof one CSI process, e.g., PMI, from the first TP can be treated asconfiguration of interferer during the CSI calculation for another CSIprocess (i.e., for another TP). This indication may be used to improvethe CSI accuracy assuming advanced receiver, e.g., SIC.

Because NR uses a flexible CSI framework, a preferred approach tosupport NC-JT CSI feedback may be a mixture of the first two optionsabove. Thus, a particular CSI framework may include TRPs or antennapanels that transmit their CSI-RS in separate CSI resources rather thanwith different ports in the same CSI-RS resource, because the CSI-RSfrom different TRPs cannot be assumed to be QCL, and further, a QCLindication between CSI-RS ports and DMRS ports for each TRP may beneeded. To indicate QCL with corresponding DMRS, each TRP may transmitseparate CSI-RS resources.

Different TRPs may be equipped with different antennas (e.g., having adifferent number of ports or port layouts). Therefore, following theOption 2 above directly may be cumbersome, because a variety ofdifferent codebooks for multi-TRP would have to be defined. Rather, itis simpler to define a separate codebook for each TRP that is appliedwith a CSI-RS resource associated with the TRP, rather than aggregatingthe CSI-RS resources and applying a joint codebook. TRPs participatingin NC-JT can be equipped with different antennas and require differentcodebooks.

Another consideration is that even if the UE can recommend a NC-JTtransmission, the recommendation may not always be optimal, because theUE may desire a low rank transmission. Thus, the UE may be able todynamically indicate how many TRPs it desires to participate in theNC-JT (including single point transmission hypothesis). Accordingly, thenetwork may configure a maximum number of TRPs/CSI-RS resources and theUE may dynamically select a number of CSI-RS resources to be included inthe CSI report. It is beneficial for the UE to dynamically select howmany TRPs shall participate in the NC-JT

To calculate CSI for a NC-JT hypothesis, the UE can select a number ofCSI-RS resources. Each CSI-RS resource can be associated with a precodercodebook (e.g., a port layout (N₁, N₂)). For each of the selected CSI-RSresources, the UE calculates a preferred precoder matrix from theassociated codebook, under the assumption that transmission occurs fromall of the CSI-RS resources simultaneously. The resulting rank the UEshall select for the hypothetical transmission is thus the sum of theper-resource ranks: ν_(TOT)=Σ_(k=1) ^(K)ν_(k), where ν_(k) is the rankof the precoder hypothesis for selected CSI-RS resource index k and K isthe number of selected resources.

The UE makes the precoder selection on the basis that the layerscorresponding to different CSI-RS resources mutually interfere. Forinstance, if W_(k) is the desired precoder matrix of rank ν_(k) forCSI-RS resource k∈{1 . . . . K} and H_(k) is the channel estimate of theCSI-RS ports of resource k, the following effective channels is used forthe hypothetical PDSCH transmission when determining PMI and CQI:

$H_{eff} = {{\left\lbrack {{H_{1}...}\mspace{14mu} H_{K}} \right\rbrack\begin{bmatrix}W_{1} & \text{⋯} & 0 \\\vdots & \ddots & \vdots \\0 & \text{⋯} & W_{K}\end{bmatrix}} = {\left\lbrack {{{H_{1}W_{1}}...}\mspace{14mu} H_{K}W_{K}} \right\rbrack.}}$Thus, the inter-layer interference across selected CSI-RS resources maybe considered. An example of multi-resource selection is illustrated inFIG. 8.

Furthermore, each TRP may use either non-precoded CSI-RS or a set ofbeamformed CSI-RS. In the latter case, each TRP is associated with anumber of CSI-RS resources that is selected with a CRI by the UE. Thus,the selection of a CSI-RS resource within a TRP (or panel) may bedifferentiated from the selection of multiple CSI-RS resourcescorresponding to different TRPs for NC-JT.

To efficiently facilitate the differentiation, a CSI Report Setting forNC-JT hypothesis can be linked with several NZP CSI-RS Resource Settingsfor channel measurement, where each Resource Setting corresponds to aTRP. Within each Resource Setting, the UE could for example beconfigured with multiple CSI-RS resource sets, each comprising a numberof CSI-RS resources, in case the TRP is utilizing beamformed CSI-RS withresource pooling, or, on the other extreme, a single NZP CSI-RS resourcein case the TRP utilizes non-precoded CSI-RS. For each (channel)Resource Setting in the CSI Report Setting, there is an association witha precoder codebook (or simply a port layout) to use for the CSI-RSresources within that Resource Setting. The TRP selection is performedby selecting one or more of the configured Resource Settings and foreach selected Resource Setting, a PMI/RI and possibly a CRI isdetermined.

The Resource Setting selection can be performed with a HypothesisIndicator (HI). For example, a UE may be configured with a set ofpossible hypotheses for DPS and NC-JT in the CSI Report Setting, forexample, according to Table 2 where a ‘1’ indicates that a ResourceSetting is selected and a ‘0’ indicates the opposite.

TABLE 2 Example of DPS and NC-JT hypotheses Hypothesis Resource SettingResource Setting Resource Setting Indicator (HI) #1 (TRP #1) #2 (TRP #2)#2 (TRP #3) 0 (DPS) 1 0 0 1 (DPS) 0 1 0 2 (DPS) 0 0 1 3 (NC-JT) 1 1 0 4(NC-JT) 0 1 1 5 (NC-JT) 1 0 1 6 (NC-JT) 1 1 1

By configuring which hypotheses a UE is able to select from in the CSIReport Setting, the network can configure different CSI Report Settingsfor e.g. DPS and NC-JT and trigger different CSI reports for thedifferent sets of hypotheses. Another possibility is to configure only asingle hypothesis, in that case, the UE does not need to feed back anHI.

An example of how the CSI framework supports multi-TRP NC-JT CSIfeedback is illustrated in FIG. 12. Each CSI Report Setting correspondsto different sets of configured channel hypotheses.

In general, to support NC-JT CSI feedback, a CSI Report Setting can belinked with more than one Resource Setting for channel measurement. TheCSI Report Setting associates each Resource Setting with a precodercodebook. A CSI Report Setting is further configured with a set ofhypotheses for channel measurement, wherein each hypothesis selects asubset of the linked Resource Settings for channel measurement and wherethe UE selects one hypothesis from the set as part of the CSI report.For the selected Resource Settings, the UE determines PMI, RI and ifapplicable CRI for each Resource Setting jointly, on the basis that thelayers from CSI-RS resources in different Resource Settings mutuallyinterfere.

NC-JT may be supported either within a PDSCH, where the different layersof the PDSCH correspond to different TRPs, or using multiple PDSCH eachtransmitted from a separate TRP. The same CSI framework may be used forboth cases, but a difference is what codeword-to-layer mapping to assumeand how many CQIs to calculate, because e.g. a rank-4 NC-JT transmissionwhere each TRP transmits two layers each can either be mapped to 1 or 2codewords depending on if 1 or 2 PDSCH(s) is used. Thus, the UE needs toknow what is assumed. This could be configured in the Resource Setting.For CSI feedback with NC-JT hypothesis, the CSI Report Setting maycontain information on whether single or multiple PDSCH is assumed forCQI calculation.

Regarding feedback of the CSI report, it simple to consider transmissionof a single CSI feedback report containing all PMIs/CQIs, even ifmultiple PDCCH/PDSCH operation is used (i.e., CSI report is not split upinto several per-TRP CSI reports). Even if the TRPs are not perfectlysynchronized and, for example, manage their own HARQ buffers requiringindependent HARQ-ACK feedback on separate PUCCH transmissions, CSIfeedback generally is not as delay-sensitive as HARQ feedback and may beshared by the TRPs over a non-ideal backhaul link (or schedulinginformation of the uplink resource allocation of the physical channelcarrying the CSI report could be shared and the TRPs could independentlyreceive the uplink transmission). Because CSI feedback for NC-JT mostlikely is aperiodically triggered, and thus carried on PUSCH, the UE maytransmit several PUSCHs simultaneously if per-TRP CSI feedback is used,which could become cumbersome from e.g. a power control perspective andshould be avoided. Accordingly, CSI feedback for NC-JT may be containedin a single report, and not split up in per-TRP CSI reports on separatePUCCH/PUSCH transmissions.

Abbreviations used in the preceding description include:

3GPP Third Generation Partnership Project

BTS Base Transceiver Station

CoMP Coordinated Multiple Transmission Point

CQI Channel Quality Indicator

CRC Cyclic Redundancy Check

CRI CSI-RS Resource Indicator

C-RNTI Cell Radio Network Temporary Identifier

CSI Channel State Information

CSI-RS Channel State Information Reference Signal

D2D Device to Device

DCI Downlink Control Information

DFT Discrete Fourier Transform

eNB eNodeB

FDD Frequency Division Duplex

HARQ Hybrid Automatic Repeat Request

LTE Long Term Evolution

MAC Medium Access Control

M2M Machine to Machine

MCS Modulation and Coding Scheme

MIMO Multi-Input Multi-Output

MTC Machine Type Communication

NC-JT Non-Coherent Joint Transmission

NR New Radio

OFDM Orthogonal Frequency Division Multiplexing

PDCCH Physical Downlink Control Channel

PDSCH Physical Downlink Shared Channel

PMI Precoder Matrix Index

PRB Physical Resource Block

PUCCH Physical Uplink Control Channel

PUSCH Physical Uplink Shared Channel

QAM Quadrature Amplitude Modulation

QCL Quasi-Colocation

QPSK Quadrature Phase-Shift Keying

RAN Radio Access Network

RAT Radio Access Technology

RB Resource Block

RBS Radio Base Station

RI Rank Indicator

RNC Radio Network Controller

RRC Radio Resource Control

RRH Remote Radio Head

RRU Remote Radio Unit

SINR Signal-to-Interference-plus-Noise Ratio

SPS Semi Persistent Scheduling

TDD Time Division Duplex

UCI Uplink Control Information

UE User Equipment

UL Uplink

URLLC Ultra-Reliable Low-Latency Communication

UTRAN Universal Terrestrial Radio Access Network

WAN Wireless Access Network

The invention claimed is:
 1. A method performed by a user equipment for reporting channel state information in a wireless communication system, wherein the user equipment is configured with two or more channel state information reference signal, CSI-RS, resources in a CSI-RS resource set, the method comprising: selecting at least two CSI-RS resources from the CSI-RS resource set, wherein each of the at least two selected CSI-RS resources are associated to a set of spatially multiplexed layers, wherein different sets comprise different layers; determining a preferred precoder matrix for each of the selected CSI-RS resources; and transmitting a CSI report indicating each of the selected CSI-RS resources and the determined preferred precoder matrix for each of the selected CSI-RS resources wherein a first one of the selected CSI-RS resources comprises a first set of layers and a second one of the selected CSI-RS resources comprises a second set of layers wherein the first set of layers and the second set of layers mutually interfere.
 2. The method of any one of claim 1, further comprising: calculating a channel estimate for each of the selected CSI-RS resources; determining a channel quality indicator, CQI, corresponding to a hypothetical transmission on a plurality of effective channels, where the effective channels depend on the preferred precoder matrices and channel estimate for each of the selected CSI-RS resources, and where layers transmitted through the effective channels mutually interfere; and wherein the CSI report further indicates the determined CQI for each of the selected CSI-RS resources.
 3. The method of claim 1, wherein each CSI-RS resource is associated with at least one of: a number of antenna ports (P); a multi-panel antenna array port layout parametrized by a number of vertical panels (M_(g)) and a number of horizontal panels (N_(g)); and a precoder codebook.
 4. The method of claim 1, wherein each CSI-RS resource is associated with a different quasi co-location, QCL, assumption.
 5. The method of claim 1, wherein the CSI-RS carried in each CSI-RS resource of the set of CSI-RS resources is transmitted from different antenna subsets.
 6. The method of claim 5, wherein the different antenna subsets belong to different transmission points.
 7. The method of claim 1, further comprising: obtaining a CSI report configuration comprising possible hypotheses for combinations of one or more CSI-RS resources; wherein: selecting the at least two CSI-RS resources comprises selecting the at least two CSI-RS resources according to a selected one of the possible hypotheses; and the CSI report indicates the selected CSI-RS resources by indicating the selected possible hypothesis.
 8. The method of claim 1, wherein transmitting the CSI report comprises transmitting a first message associated with one of the selected CSI-RS resources and transmitting a second message associated with a second one of the selected CSI-RS resources.
 9. The method of claim 8, wherein the first message is transmitted to a first transmission point and the second message is transmitted to a second transmission point.
 10. The method of claim 1, wherein the determined preferred precoder matrix for at least one selected CSI-RS comprises a first preferred precoder matrix for a first subband; the method further comprises determining, for the at least one selected CSI-RS resource, a second preferred precoder matrix for a second subband; and wherein the CSI report indicates the first and second preferred precoder matrix for the at least one selected CSI-RS resource.
 11. The method of claim 1, further comprising receiving an indication of a codeword-to-layer mapping for use when determining the CQI.
 12. The method of claim 1, wherein: a set of demodulation reference signal, DMRS, antenna ports are numbered between 7-14; a set of CSI-RS antenna ports are numbered between 15-31; and a set of physical downlink shared channel, PDSCH, signals on antenna ports {p_(k), p_(k+1)}, where p_(k)=7+Σ_(l=1) ^(k−1)v_(l), result in signals equivalent to corresponding symbols transmitted on antenna ports {15, . . . , 14+P_(k)} corresponding to CSI-RS resource k in a set of K CSI-RS resources, as given by [y⁽¹⁵⁾ . . . y^((14+P) ^(k) ⁾]^(T)=W_(k)[x^((p) ^(k) ⁾ . . . x^((p) ^(k+1) ⁾]^(T) where x⁽⁷⁾, . . . , x^((6+v) ^(TOT) ⁾ with v_(TOT)×Σ_(k=1) ^(K)v_(k) is a vector of symbols, where each symbol corresponds to a layer to be transmitted to the user equipment and contains all layers transmitting the PDSCH.
 13. The method of claim 1, wherein the determined preferred precoder for a CSI-RS resource with index k of the selected CSI-RS resources is a matrix W_(k) that is determined in accordance with a transmission vector y being equivalent to using W_(k) on a set of layers in a vector x of physical downlink shared channel, PDSCH, signals transmitted to the user equipment according to y=W_(k)x, wherein y is a transmission on antenna ports corresponding to the CSI-RS resource with index k.
 14. The method of claim 1, wherein: the PDSCH signals are on a set of v_(k) demodulation reference signal, DMRS, antenna ports that are indexed starting with a first port number D₁; the CSI-RS resource with index k comprises P_(k) antenna ports, and has a first antenna port with index C₁; and the PDSCH signals on antenna ports {q_(k), q_(k+1)}, where q_(k)=D₁+Σ_(l=1) ^(k−1)v_(l) , result in signals equivalent to transmission on antenna ports {C₁, . . . , C₁+P_(k)} corresponding to CSI-RS resource k, as given by [y⁽¹⁵⁾ . . . y^((14+P) ^(k) ⁾]^(T)=W_(k)[x^((p) ^(k) ⁾ . . . x^((p) ^(k+1) ⁾]^(T).
 15. A user equipment capable of reporting channel state information in a wireless communication system, wherein the user equipment is configured with two or more channel state information reference signal, CSI-RS, resources in a CSI-RS resource set, and wherein the user equipment comprises processing circuitry operable to: select at least two CSI-RS resources from the CSI-RS resource set, wherein each of the at least two selected CSI-RS resources are associated to a set of spatially multiplexed layers, wherein different sets comprise different layers; determine a preferred precoder matrix for each of the selected CSI-RS resources; and transmit a CSI report indicating each of the selected CSI-RS resources and the determined preferred precoder matrix for each of the selected CSI-RS resources wherein a first one of the selected CSI-RS resources comprises a first set of layers and a second one of the selected CSI-RS resources comprises a second set of layers wherein the first set of layers and the second set of layers mutually interfere.
 16. The user equipment of any one of claim 15, wherein the processing circuitry is further operable to: calculate a channel estimate for each of the selected CSI-RS resources; determine a channel quality indicator, CQI, corresponding to a hypothetical transmission from a plurality of effective channels, where the effective channels depend on the preferred precoder matrices and channel estimate for each of the selected CSI-RS resources, and where layers transmitted through the effective channels mutually interfere; and wherein the CSI report further indicates the determined CQI for each of the selected CSI-RS resources.
 17. The user equipment of claim 15, wherein each CSI-RS resource is associated with at least one of: a number of antenna ports (P); a multi-panel antenna array port layout parametrized by a number of vertical panels (M_(g)) and a number of horizontal panels (N_(g)); and a precoder codebook.
 18. The user equipment of claim 15, wherein each CSI-RS resource is associated with a different quasi co-location, QCL, assumption.
 19. The user equipment of claim 15, the processing circuitry further operable to: obtain a CSI report configuration comprising possible hypotheses for combinations of one or more CSI-RS resources; wherein: the processing circuitry is operable to select the at least two CSI-RS resources by selecting the at least two CSI-RS resources according to a selected one of the possible hypotheses; and the CSI report indicates the selected CSI-RS resources by indicating the selected possible hypothesis.
 20. The user equipment of claim 15, the processing circuitry further operable to receive an indication of a codeword-to-layer mapping for use when determining the CQI.
 21. A method performed by a network node of a wireless communication system, the method comprising: transmitting, to a user equipment, a first channel state information reference signal, CSI-RS, in a first CSI-RS resource of a set of at least two CSI-RS resources from a first antenna subset and a second CSI-RS in a second CSI-RS resource in the set of at least two CSI-RS resources from a second antenna subset, wherein the first and second antenna subsets comprise a first and second set of spatially multiplexed layers, respectively, and wherein the first and second set of layers are different; and receiving (956), from the user equipment, a CSI report comprising a first preferred precoder matrix associated with the first CSI-RS resource and a second preferred precoder matrix associated with the second CSI-RS resource wherein a transmission associated with the first set of layers and a transmission associated with the second set of layers mutually interfere.
 22. The method of any one of claim 21, wherein the CSI report further comprises a channel quality indicator, CQI, associated with each of the first and second preferred precoder matrices.
 23. The method of claim 21, wherein the first and second antenna subsets belong to different transmission points.
 24. The method of claim 21, further comprising transmitting, to the user equipment, a CSI report configuration comprising possible hypotheses for combinations of CSI-RS resources.
 25. The method of claim 21, further comprising transmitting, to the user equipment, an indication on a codeword-to-layer mapping for use when determining the CQI.
 26. A network node of a wireless communication system, the network node comprising processing circuitry operable to: transmit, to a user equipment, a first channel state information reference signal, CSI-RS, in a first CSI-RS resource of a set of at least two CSI-RS resources from a first antenna subset and a second CSI-RS in a second CSI-RS resource in the set of at least two CSI-RS resources from a second antenna subset, wherein the first and second antenna subsets comprise a first and second set of spatially multiplexed layers, respectively, and wherein the first and second set of layers are different; and receive, from the user equipment, a CSI report comprising a first preferred precoder matrix associated with the first CSI-RS resource and a second preferred precoder matrix associated with the second CSI-RS resource wherein a transmission associated with the first set of layers and a transmission associated with the second set of layers mutually interfere. 